Welcome to Kassadin, are you thinking with Riftwalk?

This is a story about a solo mid Kassadin building as a heavy AP burst champion, which is convenient because that is pretty much the only popular use for the champion right now. I will cover diverging builds, but this guide only makes sense if you assume you're leaving the character selection screen with the notion of playing as a solo mid AP carry.

So without further adeiu, lets meet the man of the hour...

Kassadin is a ranged AP champion with high burst damage potential. He uses mana as a resource for all of his abilities, and has a melee autoattack. Most of his damage comes from Null Sphere and Force Pulse, however he can eke out extra damage if he can get in melee range of a target.

Kassadin excels at assassinating sensitive targets during a teamfight, performing ganks while laning, and cleaning up weak targets after a teamfight. Most of his strengths are derived from his Riftwalk ultimate, which allows him to frequently blink around the battlefield unlike any other champion.

HOW TO READ:
- Everything above and below is nothing more than my humble theory of Kassadin based on loose reasoning, experience, and the input of others. Kassadin is my favorite, best champion, but these are by no means expert opinions.
- My sections are mostly stand-alone, with some crossover elements, so skipping sections or reading them in a different order is fine, but it makes a bit more sense together :)
- I'm probably going to continue to modify and refine this guide, so expect some changes and additions. I'll take this statement out when I'm definitly done changing it.
- I welcome any feedback, comments, questions, ratings, and discussions. I can be reached at everybin@gmail.com for any reason, even if its just to bounce Kassadin ideas around.

Void Stone: The main feature of this passive is the 15% magic damage reduction, though since you cannot really take very much damage to begin with, the attack speed component does not mesh very well with this particular Kassadin build. It does, however, have its uses while chasing down an enemy after a skirmish, or brawling with other AP carries (it happens).


Null Sphere: This is Kassadin's favorite nuke and part of his primary combo. The travel time is important, as it delays the silence briefly. Normally your opponent will not be able to react in this brief window, so it actually works to your advantage, causing your silence to cover a more important time interval. You can also use this delay to run out of turret range after poking.


Nether Blade: This is a situational damaging ability, farming tool, and utility spell. Use the passive effect to sustain and regain mana while laning and farming. It is not critical for your mana to land champion melee swipes, indeed you rarely will hence the situational nature of the damaging aspect. You can use the active component to buffer a stack of Force Pulse if needed, or to aid in last hitting minions or breaking towers.


Force Pulse: This is the other half of your primary combo. The range at which it registers ability casts is quite large, so you'll find you can use this almost on cooldown during fights once you exit the laning phase. Note that the reach of this ability is slightly greater than your Null Sphere.


Riftwalk: This is Kassadin's characteristic ability. The most important aspect of it is the movement, not the damage. Do not accumulate too many stacks, it is best to stay at or below 2 except in emergency situations (lower stacks is almost always preferrable, since the ramping up damage is not very useful in comparison to the cost). This ability will be talked about at length throughout the guide as it is the key to playing Kassadin. Keep in mind that there is a brief delay before the teleport takes effect, but once it does the transition is instantanious.

Pre-minion
A Kassadin before level 6 is like a bird without wings. The analogy is closer than you think. I take Boots with 3x Health Potion as my first items before heading up to guard my allied jungle. If you are invading or defending an invasion, make sure you cannot be nuked down, throw your Null Sphere on your team's target, and use Void Stone to hopefully continue to deal large amounts of damage after everyone's ability is on cooldown.

Helping the Jungler
If your jungler requests aid you can probably afford to help with a single camp. Don't use more than one Null Sphere, and prefer not to help with the Blue Golem or Red Lizard. You have a melee range and need to get to level 6 as soon as possible.

Laning
When laning begins, land as many minion last hits as you can without succumbing to pressure. Thankfully Null Sphere can allow you to trade damage decently. Use it to prod more powerful champions back after they've played their hand, and use the space to take last hits. Do NOT try to make anything 'happen' before Riftwalk unless your opponent over extends.

You are extremely susceptible to ganks at this point in the game. Conserve your Flash and do not push your lane.

Keep in mind that the threat of landing a Null Sphere is generally more useful at this point than the ability itself. When given a choice, choose not to trade health evenly. Your mana pool will not be able to handle constant use, and it will make you ever more susceptible to ganks.

Once you obtain Force Pulse you will be able to pair it will Null Sphere when available. Use this combo to simply keep poking and landing last hits, nothing fancey. These abilities work best if you use them in quick succession. Force Pulse has a slightly longer range, but also applies a slow, so as a rule of thumb, if your target is in range of Null Sphere use it first, then Force Pulse. If he/she is just out of range, use Force Pulse first for the slow, then catch up for a Null Sphere.

When performing your own ganks with your jungler, do not modify your behavior. Ideally it should already be very passive, so just let your opponent lean into you.

Use your Nether Blade active to assist in landing last hits at opportune moments. Its mana cost is fairly small. In addition, Null Sphere can be used to last hit if you have some leftover mana and plan to Recall soon anyways.

Shopping
Although it is not always possible, try to avoid returning to base until you have 750 gold. This will allow the completion of Sorcerer's Shoes which you will need to start making things happen at level 6. Once you hit 6, provided you have 750 gold, it is a good idea to just safely trade some damage on the opponent and return to base for this item.

Once you obtain Riftwalk and Sorcerer's Shoes, all the options open up to you for a time. You can easily push most champions out of any lane, or participate in any number of ganks. Furthermore, you become extremely resistant to almost everything enemy champions can do. I will explain it all in detail here.

Avoidance
With Riftwalk, Kassadin is no longer burdened with notions so petty as "position", whereas most other champions are still very much restricted to laning tactics. This is the root of Kassadin's impunity in the late laning phase. By moving laterally through the river and enemy bank (presumably to pressure or gank other lanes) you can safely zone enemies out, restricting their possible locations to a smaller area. Using this knowledge and a little foresight, you can escape most any threat by using Riftwalk to climb the river bank.

Buff Up
Ask an ally to help you obtain the Blue Golem buff just as you reach level 6. You will need this to truely dominate this pahse of the game. This acts as a stand-in for the Athene's Unholy Grail you have yet to obtain, and will likely be the best use of the blue buff all game long.

Pressuring your Lane
Kassadin can pressure almost any single laner back to hug their tower or Recall, the secret is a gimmicky combo:

Step 1. Riftwalk just into Null Sphere range.
Step 2. Unload Null Sphere and Force Pulse immediately.
Step 3. Turn and run immediately.
Step 4. Repeat when possible

Why does this work? Normal human reaction time is too slow to allow the enemy champion to get an ability off before the Null Sphere lands since you just appear there with no warning. On the other hand, if you walk up to the enemy, they will be able to instigate the act of casting their own harass before you can silence them. Because Force Pulse applies a slow, they will not be able to get back into harass range of you before the silence wears off. They have no choice but to back off when you land this combo, and that is what ~90% of victims will do.

Making the Kill in Lane
As explained above, your pokes in the late laning phase are pretty devastating. They are also hard to avoid save for tower camping because of the combined reach of your ranged damage combo and your Riftwalk. If your opponent refuses to Recall once you have reduced their health, it is a good idea to Flash (or less ideally, Riftwalk) up to them, plant Ignite and unleash your ranged combo, following up with a Riftwalk onto them and Nether Blade melee swipes if possible

Leaving your Lane
For the reasons in the above section on avoidance you can freely move between lanes at this phase of the game, giving you and your team a window wherein you essentially have map control. With Sorcerer's Shoes you can execute some pretty exquisite ganks. Try leaping from an unseen (and perhaps unexpected) location with Riftwalk into melee range of your victim, drop your Force Pulse and Null Sphere on the victim, activate Nether Blade, and proceed to lash your victim until it dies. Don't be afraid to tower dive a bit with a second Riftwalk.

As always, only leave your lane to perform ganks if you can do so without allowing your tower to come under siege. Try poking your laning opponent extensively, then roaming after they Recall.

Kassadin is a very strong champion through the middle game, especially if guerrilla-style teamfights are taking place in the jungle. He is less useful in compact teamfights, and in solo situations, but he can still be quite deadly.

In the jungle you will have a very powerful mobility advantage because you can repeatedly hop over cliffs while opposing champions doing this will incurr substantial cooldowns. For this reason, try to make your fights happen in the jungle.

Compact Teamfights
Keep these at arm's length. the best you can hope for in a dense teamfight is to hit multiple enemies with Force Pulse and Null Sphere an important target, then stay way out of range until the scatter or chase phase of the fight.

Once the initial explosion of damage and abilities ends, you can take a larger role in the teamfight. Try to anticipate enemy escape routes and head them off with Riftwalk before they escape.

Guerrilla Teamfights
This is truely where Kassadin shines. If a teamfight takes place in or around the jungle, and is reasonably spread out, Kassadin can maximize his presence. Since the enemy team will not be together, it is nearly impossible to damage Kassadin efficiently in the space of one ranged disable or one cooldown period of Riftwalk, especially since he will have likely just erased ~40% of your maximum health with his combo.

In these situations, you want to be as aggressive as possible without over extending. Enemies may be able to trade health evenly with your Kassadin, but they cannot trade positional advantage with him whatsoever. More on this later.

Duels
When engaging an opponent 1v1 as Kassadin, it is important to remember that you have very low damage per second output as compared to your survivability, so you need to avoid brawls. Consider following your target around, waiting for cooldowns at a golden distance where you are far enough to escape unscathed from anything your target might do with a single Riftwalk, but close enough to Riftwalk into Null Sphere range. Most champions have such a distance you can keep them at.

Warding, Dragon, and Buffs
I would recommend asking your allies to spend the gold for Sight Ward and Vision Ward until the late game or late middle. You are on a clock to make good use of Rabadon's Deathcap.

Do not attempt to solo the Dragon, as you lack the survivability, sustainability, and damage per second.

The Blue Golem buff is very efficient for Kassadin, take it at every available opportunity all game.

Kassadin becomes less effective after a certain point. This is partially due to his lethality dropping off as enemy units build survivability items late game, and partially due to the increased frequency of compact team fights. Make the best of this phase by using your unique talent to muscle for position for your team.

Of course you can always continue to be deadly with Kassadin, right up until everyone is maxed on items, but it requires a substantial gold lead that becomes increasingly difficult to maintain. Indeed this build is designed to chase that steep curve.

Teamfights
Execute teamfights in much the same way as you would handle them in the middle game, but be aware that tanky champions may not make good targets. Ranged AD Carry champions can be unfavorable to fight alone as well at this point, as they can kill you very quickly despite being silenced, and usually recover most of the damage you did through lifesteal.

Note that at this point you should have a Lich Bane, which adds a new dynamic to Kassadin. Choose wisely when to enter melee range with your opponents. Combining Lich Bane with Nether Blade, Void Stone, and the damage from a stacked Riftwalk can be a devastatingly strong follow up to your basic combo, but it makes you vulnerable. Think about what disables can be used on you before you use your melee range damage.

This Kassadin build leaves you very squishy, especially against physical damage. Generally this late in the game, my Kassadin survives by never engaging an opponent thats in a position to deal damage to you. Note that if you can finish an enemy with one round of your cooldowns, they can't deal any damage to you :) This being said, Kassadin can survive a moderate amount of magic damage due to the high magic resist from my build along with the innate magic resist from Void Stone. Use this to hard counter the enemy AP Carry in the late game.

Pushing
The damage bonus from Lich Bane and Nether Blade applies to turrets, making Kassadin a better than average tower killer. Make use of this and the security of Riftwalk and Flash to bully the enemy team into unfavorable engagements.

It is also possible, but sitiuational to pick off an enemy Inhibitor, then retreat through the jungle with multiple Riftwalk casts.

Warding, Dragon, Baron, and Buffs
Feel free to take over the burden of buying Sight Ward and Vision Ward in the late game, especially if you are going to max out on items before your allies, and its that kind of a game.

You CAN solo the Dragon in the late game, but it will take you a while, and eat a good chunk of your health, so I would not especially recommend it. In a related note, when your team is killing Baron, use Nether Blade to contribute damage while conserving your more impactful abilities to assist if there is an enemy incursion.

Continue to seek out the Blue Golem buff all game.

Health Potion: I take 3 of these along with Boots as my starting items. The Boots reduce vulnerability to early ganks, and sometimes make it easier to sneak into melee range for minion last hits. The reason I take Health Potion is that it is the safest way to ensure that I can stay in my lane until I acquire 750 gold. If you think you can get the 1100 gold required for Sorcerer's Shoes around level 6, and before your first Recall with a different item choice tailored to your laning opponent, go for it.


Sorcerer's Shoes: My goal is to obtain this item by the time I hit level 6, or Recall for the first time. The flat spell penetration really augments your early killing power, which will enable your ganks. The movement speed will enable you to keep up with the fleeing enemies you gank so you can take out the middle portion of their health pool with Nether Blade melee attacks.


Athene's Unholy Grail: This is the next item I work towards. The mix of cooldown reduction, mana regen, and ability power hits the spot for a midgame Kassadin, allowing sustained, frequent, and powerful ability usage. The 12% mana on kill or assist will sustain frequent Riftwalk usage while cleaning up after teamfights. The magic resist begins to develop Kassadin as an anti-caster. If laning breaks down early, start with Fiendish Codex, but if laning remains coherent, you will first want the sustain of Chalice of Harmony.

This is probably the most unorthodox aspect of my Kassadin build. Most players will insist on building a large mana pool quickly with Tear of the Goddess or Rod of Ages. While I respect both of these choices, and the need for Kassadin to have large amounts of mana, the mana regen from this item can act as a reasonable subsitute. The strongest reason I have for basing my Kassadin build off this item is that it allows early cooldown reduction while filling all of his other early needs. In short, this item in place of the other popular choices allows Kassadin to develop faster.


Rabadon's Deathcap: Once you have your Athene's Unholy Grail your usefulness to your team in the mid and early late game will be mostly dependant on how much of an enemy health pool you can take off with a Null Sphere + Force Pulse combo. For this reason the radical ability power scaling of Rabadon's Deathcap and its precursor items becomes a very attractive pursuit. I look to obtain this item as early in the midgame as possible. This can win games.


Lich Bane: As the late game begins to develop, a strong Kassadin will need to make use of his melee output as well as the standard burst combo in some situations. I make this transition by making a Lich Bane as soon as my standard burst starts to drop off. I prefer to make this item after Rabadon's Deathcap, but making it after Void Staff is also a good option if you feel you need to continue working on your primary damage output. Also, if you find yourself facing strong attack damage brawler champions, you may want to skip this item for now.

and
Void Staff / Abyssal Scepter: I take these items to keep up with the increasing survivability of tanky champions through the late game, picking one up either immediately before or after Lich Bane, depending on the risk of comitting by entering in melee range, as well as the timing of the magic resist and health scaling of the enemy team. Take Void Staff if important targets (and to a lesser extent tanky targets) are taking alot of magic resist, otherwise make an Abyssal Mask first.


Enchantment: Furor: The boot upgrades are not stellar for Kassadin, but Furor is worth getting at the end so that you can maneuver around enemy champions, positioning for your next Riftwalk. Obviously get Homeguard instead if your base is being decimated at this point in the game.

Banshee's Veil: This is a great choice for survivability, especially if ranged crowd control is locking you out of teamfights. The mana does not go to waste, though it is not a crititally important stat at this point, and the health and magic resist will make you a nightmare for the enemy AP carry at this point in the game. You can take this in place of Void Staff or Abyssal Mask if you think defense is at all needed. This is probably the most common item subsitution I will make.


Zhonya's Hourglass: This is a decent last item as a middle ground between defense and offense. The 100 ability power attempts to keep up with defensive scaling, while at the same time granting Kassadin some clutch survivability. All in all, this Kassadin build uses very little direct survivability stats, and relies primarily on magic resist and the ability to escape quickly with Riftwalk to avoid death. The extra 50 armor will not help with the survival of exposure to ADC champions, but it will make getting in melee range of offtanks more palatable. The active effect on Zhonya's Hourglass is very useful for surviving comittments because the grace period will bring you closer to being able to Riftwalk away to safety.

or
Rod of Ages or Seraph's Embrace: These can be suitable middle game items, but ultimately I feel that they do not belong in this particular build. The large amount of mana is not especially needed since I am using Athene's Unholy Grail, which also sports the critical cooldown reduction bonus. There is definitly a legitimate argument to be made for other, more mana pool based builds, but this would change the entire face of the item build if included.


Crystalline Flask: Picking up one of these along with some combination of wards and potions can be a decent starting build. If you take this, you're going to have more sustain, and be able to stay in your lane for longer. Furthermore the Crystalline Flask will continue to give you a nice boost to your sustain during the midgame. I would recommend this when laning against a champion that is difficult for Kassadin to stay in lane against, such as Teemo.


Deathfire Grasp: Ok this is an amazing item, but because the cooldown reduction is redundant I don't feel it belongs in this particular build. This would be better listed as a standard item in a build that utilized Rod of Ages or Tear of the Goddess as a core item. Nevertheless, if your team is being dominated by one champion in particular, this COULD still be a worthwhile choice. If you do not have the scaling cooldown reduction glyphs (which I like for the midgame), this could be a good mid-late game item in place of Abyssal Mask or Void Staff.


Mejai's Soulstealer: Kassadin is largely a 'snowball' champion, so this item would seem to make sense, but unless you completely outclass the enemy team, you will die at some point, and this choice will come back to haunt you. If you do completely outclass the enemy team, then this is a risk you don't need to take.


Morellonomicon: This could be argued as a replacement for Athene's Unholy Grail, but I find that the grail can act as an independant mana sustain unit, whereas using the tome would necessitate the use of another mana based item. Furthermore, the healing reduction is not terribly useful for Kassadin since he will be bursting champions, and not usually focusing them with sustained damage per second that champions will be attempting to outrun with healing abilities.


Liandry's Torment: The ability power and magic penetration of this item make it attractive, but with only 200 health and a passive that is useless to Kassadin, it is always better to choose Abyssal Mask over this item. The reason the passive is useless is that this Kassadin build will take off the top portion of your target's health any time he does damage, so by the time the damage over time effect kicks in, it will be operating on a relatively insignificant amount of health.

or
Frozen Heart / Iceborn Gauntlets: Some apsects of these make them nice defensive items. Mana and cooldown reduction are right in Kassadin's ballpark, but building Kassadin's defense against physical damage is not really a worthwhile pursuit. If you take one of these items, you will need to sacrifice Athene's Unholy Grail for a Rod of Ages, and possibly drop Lich Bane. Furthermore, you will need additional anti-AD items to make this worthwhile. This being said, if you get stuck against a very AD heavy team due to poor fortune, I would consider Iceborn Gauntlet in place of Lich Bane and Rod of Ages in place of Athene's Unholy Grail


Doran's Ring: This is a decent starting item, but it will be a challenge to get the full 1100 gold you need for Sorcerer's Shoes before your first recall. The stats definitly help to juice up your early game, but unless your team is planning an invasion, I'd stay away from this.


Mercury's Treads: This is an ok idea as an endgame item, once you are otherwise maxed on items and have an Elixir of Brilliance. I will always take Sorcerer's Shoes as my boots because it is of tremendous benefit early game as you are ganking lanes.


Rylai's Crystal Scepter: This seems like a strong choice, but the slow is essentially wasted. Force Pulse ensures that your victims are slowed when they need to be.


Will of the Ancients: Kassadin has relatively low sustained damage per second, and as such spell vamp cannot contribute to making Kassadin more able to brawl. Indeed, this build is not set up to survive extended exposure to damage anyways. Don't get this with this build.


Guardian Angel: This is not a strong option for Kassadin since once he revives he is unlikely to be able to Riftwalk to safety before being picked off because of the large delay to the revive.


Warmog's Armor: You don't need to deal sustained damage, so you don't need to take sustained damage... and even if you did, this would not help you do it.

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Primary Summoner Spells

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Flash: At first glance Flash may seem redundant because of Riftwalk, but on the contrary this summoner spell synergizes very well with its low-cooldown cousin. This ability has many applications throughout any game, but perhaps most importantly it adds directly with the reach of your Riftwalk. This means you theoretically cover a much larger circle of the map at any given time if your Flash is off cooldown. Moreover, if you get caught in a deadly situation and your Riftwalk (which you will be using frequently for various purposes) is unavailable, trusty Flash can save your life.


Ignite: As with most other AP Carry champions, Ignite will help you close out your burst sequences, and make the difference between landing a kill, and missing it. Since Kassadin snowballs heavily, and specializes in assassination, getting those extra early kills is very much worth a summoner spell.

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Other Summoner Spells

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Cleanse: This is a very strong late game summoner spell for Kassadin if the enemy team has any long duration full disables, especially those that act at a range. Disabling Kassadin's movement is the best way to kill him, and his primary counter in the mid and late game. It is painful to give up Ignite or Flash, but Cleanse is all but a necessity under certain team compositions if you plan on being useful in the late game. Here are some examples of champions I would insist on taking Cleanse against:

Ashe, Fiddlesticks, Leona, Amumu, Annie, Malzahar, Morgana, Lux, Ryze, Veigar, Brand, Zyra, Maokai...


Clarity: Kassadin is very mana reliant, and this summoner spell goes a long way to fixing that problem. Taking this can allow you to be very aggressive where you otherwise could not, and can be used to enhance your strategies and map presence. This can be better than any other summoner spell for Kassadin, but you need to know exactly how you plan on using it before taking it (i.e. you need to be a part of a coordinated team to make good enough use of it).

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My Rune Choices

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Scaling Cooldown Reduction: I don't take these for their effect at level 18, more for their effect in the level 9 - 15 range. As you know from my choice of Athene's Unholy Grail, I consider having high cooldown reduction values through the midgame very important. Kassadin's damage in the midgame is based almost entirely on the cooldown of Null Sphere and Force Pulse, and you will be attempting to utilize each of these almost on cooldown. In addition, Kassadin's mobility scales off cooldown reduction. Reducing the time between Riftwalk casts in a tense situation is crucial to positional strength, and survivability at the same time.


Gold Per 10 Seconds: Ok, so seals are really not very interesting for Kassadin in general. I take this to help supplement the gold loss from roaming around during the late laning phase, and because it is perhaps the most directly useful of the seal options. Read: I'm not super confident about this choice because none of the options really make alot of sense, this one included.


Flat Magic Penetration: This is a fairly obvious choice for Kassadin's mark slots. The magic penetration stacks up nicely with the other magic penetration in this item build, and provides a hefty benefit in the early, middle, and late game.

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Other Rune Ideas

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Flat Armor: It is a good idea to keep a set of these on-hand, as they are a requirement in the unfortunate case where you end up facing an AD mid. If you are playing blind pick, ALWAYS take these. They are not a terrible choice against an AP mid either, but I consider their benefit pretty limited.


Flat Health Regen: This is my second choice when facing a generic AP champion in the mid lane. Kassadin does not otherwise possess any health sustain, and this can sometimes help him stay in lane when he needs to. Kassadin will always take some damage while pursuing minion last hits because of his melee attack, so this will not go to waste. Furthermore, this is not a terrible choice for when the laning phase ends, as it will make damage just a tad less sticky throughout the game.


Scaling Ability Power: This is a good choice if you do not like my cooldown reduction prioritizing build, or if you choose to rely more heavily on the blue golem buff to be effective.


Flat Magic Resist: This makes alot of sense in the laning phase, but sacrificing the cooldown reduction for the mid game is not something I'm willing to do.


Magic Penetration Quint: This is a pretty solid substitute for the cooldown reduction quintessences, and I can't really argue with it. It is without a doubt stronger in the early game, and remains strong in the middle and late game.

Middle Lane
In general, Kassadin is very weak to AD mid champions. This is partly due to his Void Stone, and partly because his silence does not stop AD champions from dealing significant damage in general. Since Kassadin is balanced around these abilities, he comes up quite short against AD champions with any reach. Don't play him if you know you're facing an AD mid.

In addition, there are a number of counters to Kassadin, but similarly a number of champions Kassadin counters pretty heartily:

Kassadin Counters in Mid:
Fiddlesticks, Annie, Karthus, Kennen, Syndra, Veigar, Zilean

Counters Kassadin in Mid:
Talon, Urgot, Fizz, Gangplank, Pantheon, Teemo

Make a special note of Talon and Fizz, as they are very hard counters to Kassadin, and you should avoid laning against them at all costs.

Enemy Team Compositions
Kassadin is much more resistant to magic damage than he is to physical. As a result, he is quite strong against AP heavy teams, and quite weak against AD heavy teams. Consider that if the enemy team lacks physical damage outside their ranged AD carry, Kassadin will thrive.

Kassadin prefers to engage enemy teams that are spread out as opposed to compact. If the enemy team's composition indicates they'll sticking together, Kassadin might not be the right choice.

Perhaps most importantly, Kassadin is quite weak to disables, especially ranged, hard disables. Consider that anything which can prevent the use of Riftwalk can allow the enemy team to destroy his health bar in seconds. Try to avoid using this champion if the enemy team has many such abilities. Notice that snares and silences will prevent the use of Riftwalk, but slows will not.

Allied Team Composition
Kassadin Synergizes well with allied teams which tend to force enemy teams apart, or into guerilla situations. Consider that Kassadin may be a strong choice to accompany a team with alot of AoE damage or crowd control. Kassadin is very much an assassin by nature, so make sure you are not overdoing the number of assassins on your team.

A team with good pokes can force enemy teams into positions where Kassadin can make opportunistic strikes.

Individual Champions
Some champions that belong in different lanes than Kassadin can still weigh into your decision to play him or not. The unique ability Riftwalk helps to neutralize a number of champions.

Kassadin Likes to Fight:
Singed - Can't do much of anything to Kassadin, or even challenge his positional strength
Garen - Has no tools to even get near Kassadin
Katarina - You have a good chance of stopping her ulti in a teamfight
Leona - Upon being hit by her sword, teleport to decide her landing position yourself
Blitzcrank - You can easily bait / avoid his grab, and catch up to him for the kill
Trundle - Needs to fight on his turf, which Kassadin can easily avoid
Nidalee - Kassadin is one of the few champions than can catch and kill her in the jungle
Jarvan IV - His ulti is nothing to Kassadin, and in many cases will help him out greatly
Miss Fortune - You can teleport out of her stuff pretty easily

Kassadin Dislikes to Fight:
Lee Sin - His closer is a direct counter to Kassadin, and he kills him very quickly
Nocturne - Can often catch and kill Kassadin despite his teleporting
Evelynn - She can keep up in pursuit with Kassadin with her speed boost
Malzahar - If you get caught by his ulti with enemies nearby, you're dead