Hello I'm diamond player from EUNE. I main assassins and particularly got myself to diamond playing that role. So first of all this is my first guide. I decided to do it for Talon and I'm waiting for the feedback. I will probably make more. There is some things about Talon I'm gonna bring up to you, guys. Things that are missing from some other guides.
Talon is real definition of Assassin . He is strong but still underrated and unpopular. Often people just play him bad and build bad. He is good pick even in high elo and can easily smash enemies down. I am gonna especially highlight his build, which is maybe a bit different than you used to see, but I'm gonna explain this. The reason why Talon is good is his burst as well potential snowball like madman and carry the game thereby. He is best suited for mid lane due to easy access to any lane and mid laners are usually quite squishy. He shines in mid lane and bring terror to enemies. He doesn't have that much counterpicks and he is counter almost to any mage. But even counterpicks can't deal with him when he gets ahead with right items. That's one thing. People don't really know which items are just insane on him.
Again like any other champions. He doesn't instantly win a game. Usually it is not because of certain champion in enemy side. But a team compositions that counters Talon's playstyle. To ease you, I'm gonna tell you that this doesn't happen much often in solo queue. Additionally in solo queue there is less teamwork than in ranked team games. In solo queue he can carry well if played right way.
ps. Sorry for my english as it's not my native language.

Pros

+ High damage
+ Good waveclear
+ Excels at catching people
+ INVISIBILITY
+ Escape potential
+ Kills any squishy champion instantly
+ Makes enemies hate you
+ Hard snowballing

Cons

- Some champions can prevents you from snowballing
- When behind, kinda useless
- Squishy
- All-in champion. Kill or be killed

Recommended spells
Flash Flash is absolutely best summoner spell in League of legends. It gives you mobility and some of the great plays are made with flash. Arguably flash works in every situation. It have secured so many kills for me and saved my bum thousands times. I have tried many spells to replace flash and it didn't work out for me. So I take this Flash no matter what!
Ignite Ignite is good choice for assassins. With ignite you can secure many kills, especially in early stages of the game. Also it gives advantage when you are dueling somebody.


Alternative spells

Exhaust This is a good spell. It doesn't give you kill potential as much as ignite. Although it have it's own benefits. If I pick this spell, I have to play less aggressive at lane. On the good side, I easily mitigate enemy's damage. Works well if you have high damage champion against you like Zed, Annie. It is actually better to use when enemy's team have many high damage dealers.
Ghost Good spell when you want to be Speedy Gonzales. I don't replace it with flash instead I'm choosing it over Ignite. I use this spell rarely. If you have Ghost and Flash I advise you to buy then Enchantment: Distortion
Teleport I think this is a good spell. I just personally don't like it and haven't used it that much, but maybe I just use it at wrong times. If you have some experience with Teleport take it. It gives pressure on other lanes and can give u few kills, if used at right time. It is still not that needed since for mid lane it is quite fast to get to any lane and with Mobility Boots it becomes even faster
Barrier Heal Good alternative defensive spells, but they are not really made for assassin gameplay. It reduces kill potential at lane. On the other side they might let you survive some tough situations

Versus ability power using champions
I use flat runes, because of early game boost they provide. Talon needs to survive laning phase. Most of the time you will have against poke mages. And Talon as a melee champion is prone to be poked a lot. Since armor runes were nerfed I started to use flat health instead as AP mages won't deal significant damage with their auto-attacks anyway.
As for armor penetration, I see much better options than flat AD on marks (still have AD on quintessences). Coupled with brutalizer you will have 20 flat armor penetration that's is huge boost when enemies won't have much armor in the early game.

Versus attack damage using championsI have mixed defense with armor and health. Never underestimate health, guys. Health counters magic, physical and true damage, except percentage of the health damage. So I sacrifice 1 quintessence for armor. Many AD champions have huge physical damage early on.

For the glyphs I prefer scaling cooldown reduction(CDR). That 7,5 flat CDR doesn't help that much since I'm still quite limited by mana pool. And by level 18 I will have nice 15 CDR. That's huge! But that's up to you. I have seen many people flat CDR too. Use it if, you like a bit different playstyle.

bonus AD. My specifically full build gives bonus 225 AD. So 225+25 is 245 and from this mastery I would get just +5% bonus damage which makes it about 12 AD. I think there is better options in offensive masteries. (Bonus Attack damage doesn't increases as you level up like base Attack damage do.
It's not worthy to spend on this mastery because there is much better to offer.
Fury I don't understand who use this mastery on ability focused assassin. That attack speed doesn't help you like CDR do. Most of Talon's damage comes from abilities.

Mercy
Talon's passive. Gains 10% increased damage to auto-attacks against opponents who are slowed, suppressed, immobilized or stunned. Pretty decent passive.

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Talon's Q. Activating gives you next auto-attack to deal bonus physical damage and apply bleeding on enemy. While enemy bleeds you a get vision over that enemy for 6 seconds and it works like Ignite eating enemy's health over time. Also works as reset for auto-attacks. Good for taking down turrets.
Cooldown- 8/7/6/5/4
Mana Cost- 40/45/50/55/60
On Hit Damage- 30/60/90/120/150 + .3(Bonus AD) + Auto Attack Damage
Bleed Damage(Over 6 Seconds)- 10/20/30/40/50 + 1(Bonus AD)

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Talon's W. Sends out valley of daggers in a cone which then quickly returns dealing damage on both times when passes through enemy. At laning phase W works as a poke and a farming tool. Deals lot of physical damage and applies slow on enemy. Works with Mercy. This is main ability for damage so I max it first.
Cooldown- 10
Mana Cost- 60/65/70/75/80
Range- 600 Units
Damage per Hit(Potentially hits twice)- 30/55/80/105/130 + .6(Bonus AD)
Slow- 20/25/30/35/40 %

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Cutthroat
Talon's E. Blinks instantly to the target increasing his damage on that target for 3 seconds. As well applies slow on target by 99% for 0.25 seconds. Good gap closer ability. On lane you should be careful, when you use it as enemy will probably throw crowd control on you and harass you back. Use it if enemy have used at least one of his/her abilities and activate Noxian Diplomacy just right before you E and follow up with Rake. Also works somehow as an escape ability. If you run through lanes you can use it on enemy minion to extend distance between chasing enemies.
Cooldown- 18/16/14/12/10
Mana Cost- 35/40/45/50/55
Range- 700 Units
Damage Amplification- 3/6/9/12/15 %

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Talon's R. Talon disperses blades outwards in a 500-radius ring and gains stealth for up to 2.5 seconds while gaining 40% movement speed. After 2.5 seconds or if pressing R again, stealth ends and blades return back to Talon. Deals damage when blades expands and contracts. This ability can be used in many ways. To deal tons of damage you will probably use all abilities all together. Besides that Talon's ultimate is good for escaping and engaging. Good way to engage with ultimate, is use it in the brush and run toward the enemy and use Cutthroat. They will not see that coming! Keep in mind that, you have to kill the target as you will not have any ability to escape. Due to movement speed bonus Shadow Assault is good also at catching up enemies. One more thing! You can use Flash and Noxian Diplomacy during the stealth as it will not break it.
Cooldown- 75/65/55
Range- 500 Unit Radius
Mana Cost- 80/90/100
Damage per Hit(Potentially hits twice)- 120/170/220 + .75(Bonus AD)

Fastest way to kill an enemy (unless tank) is to SPAM all abilities all at once. Use the combo (combintaion of abilities) in this order E Q W R or E R W Q if enemy tries to escape. I try to press W and R simultaneously. That deals millions of damage and the enemy won't have time to react. Combo can be extended but try to use it in 3 second at the latest because of the damage amplifier of Cutthroat which lasts just that long. Of course it is not always the wisest choice if the enemy is already half dead. For instance if at lane enemy is pretty damaged, you can easily kill him/her suing this technique. When talking about regular mage, even Assassin , you need to get them under 75 % of health and you can go for the kill. They won't except that INSANE BURST. Of course don't do it if enemy is fed and you are not!

These boots are just best for Talon. Other boots can be used in rare situation. Why these boots? Because they give that insane mobility, which makes Talon a roaming monster. After The Brutalizer and maybe Tiamat you should buy definitely these boots and start roaming to other lanes. You will get mostly like fed from other lanes. Also you will able to stay enough far away from enemies to not be focused down and when teamfight strikes you will be there in blink of the eye. Not to the mention how many kills you will gonna secure as enemies just simply will not escape from you. With ultimate and Youmuu's Ghostblade almost nobody can outrun you.

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Usually I will try to finish this item first as it gives huge damage boost. When I said first I meant as my first damage item. I usually get Mobility Boots at the time I have Tiamat. Fast wave clear let's me roam even often. And the life steal is also good especially if I'm bump into 1 vs 1 fight.

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I love this item. Nice active and the armor penetration is very useful. While 30 AD and 15 % are not seems to be big. It is enough for early game. And coupled with Infinity Edge you will start to critically strike much often. That movement speed buff helps you even better catch enemies as well as escape. This is my second damage item.

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I see this item rarely on Talon. People tends to underestimate this item. I rush this item sometimes first if I'm fed, if not the last. Very good item on Talon. Gives 80 AD, 25 critical strike chance and critical strike will deal 250 % damage. With Youmuu's Ghostblade critical strike chance rises 40 %. When you jump on enemy you will probably hit more than one basic attack. If you critically hit a squishy champion you will probably kill him/her without even using ultimate Shadow Assault. And if the enemy is tougher you will hit even more critical strikes. So underrated item. Because any other item won't give you same strenght in 1 vs 1. And I'm talking about when you didn't kill the enemy with ability combo. Oh almost forgot!! If enemies are under certain crowd control Mercy becomes even powerful when your CRITS will deal 10 % more damage.

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Must have on any AD assassin. Will not let armor stop you from murdering people. This usually my 4th item.

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Over all defensive items I prefer this item. It will give you chance to survive if you go down during killing an enemy. If you are fed, enemies will most likely save crowd control for you and focus you down. As you will be resurrecting back to life, your team will take care rest of the team. As a melee champion who goes all-in, definitely need to have at least one defensive item. Don't waste last item slot on damage, as you will have lot of damage from the items I mentioned above.

Ionian Boots Of Lucitidy
Pretty good choice, maybe by the end of the game. When you feel like you need more CDR than a mobility.

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Also not a bad choice, If enemy have lot of slows. Upgrade to Enchantment: Alacrity by the mid game in order to have enough movement speed to have it almost at same level with Mobility Boots.

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They are useful for Talon sometimes. Get it if enemy team have tons of crowd control.

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Good item, but not really recommended for Talon. Although Spellblade passive gives extra burst and Rage let's you keep up with enemies. Good passives but, you will use Spellblade probably once during combo and Rage is kinda useless if you kill enemy right away. Even the stats altogether are not really best. There is nowhere to replace Trinity Force. All items in my Core Build fits much better on Talon and with his abilities. This item is just for fun.

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This is great item on AD assassin. In my opinion this is more late game item. This is the main reason I don't buy this item so often. Armor shred is useful especially on armor stacking tanks. In the early and mid game flat armor penetration is much better as enemies won't have much armor at that time. If enemy team have enormous amount of armor then, I replace this Black Cleaver with Infinity Edge or Youmuu's Ghostblade. (Still wonder why I like so much Youmuu's Ghostblade? Scroll up!). If you don't have full AD team composition, you will deal tons of damage with my core build as enemies will not buy just armor.

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Good defensive item. Get it only if enemy have lot of crowd control or you can't dodge hooks like Rocket Grab or Your Guardian Angel is on cooldown.

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I buy this item sometimes if I'm doing poorly against some mages or assassins.

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As Talon is not front liner you won't be focused down until you actually jump on enemy in the of teamfight. Even then that active from this item probably won't save you. You get maybe from crowd control but it won't save you from damage. Banshee's Veil gives better defensive stats and Guardian Angel is 10x better.

Early Game/ laning phase

Talon is pretty weak pre-level 6. It's quite hard to kill anybody at lane if enemy won't tower dive or make any other mistake. Talon's kill potential increases when he gets his ultimate Shadow Assault. Basically at lane you just need to farm as best you can and poke if back if you can. You have to develop skill to dodge skill-shots. Talon is melee champion he will be easily zoned if you will be low on health after taking too much poke. In that case I recommend to buy Crystalline Flask. In worst scenario you might be killed and shut downed. Don't give any kill to enemy at laning phase. If you ate too much poke just go to base, keep doing that and if you cannot survive or get a kill enemy from your lane, then ROAM!! Talon does that best. Buy mobility boots fast (of course after one decent damage item like The Brutalizer )and start running around the map. If you got Tiamat that's even better as you gonna wave clear your lane faster. If you lose your first turret and you got couple of kills or even assists for than, then it is pretty good trade. Good jungler will help you against your laning opponent, but don't rely on jungler every game. Be also careful of enemy jungler. When you push too hard you might be killed as Talon's escapes are pretty limited before level 6.

About wards. Please guys and girls buy wards! This advice is mostly for low elo rank. But even in high elo games some people buy wards rarely. They are not made just for support, you need to buy them and keep vision for your team as well. Nobody gonna protect your butt when you are solo laner! Trinket Stealth Ward is not enough. Sometimes I'm second ward buyer after support. Nobody gonna protect your ***! If you dont ward, you will most likely be ganked and KILLED! When you are at laning phase grab at least couple wards once in while. Of course you need items, so buy them first. Wards are not waste of money. The other way around warding saves lives and maybe let you get a kill on stupid enemy, who made a wrong path. And at least buy 1 vision ward in every game. Let you see even enemy wards and stealth using champions. Buy wards, make it a habit.

There is one trick I like to use when playing especially Talon. When I'm still in laning phase and I got Shadow Assault, I put one ward in enemy jungle, near to jungle camp and wait for enemy jungler to come. Usually he/she won't be in full health and pretty easy to kill.

Mid Game/Roaming

After you get few items like Tiamat, The Brutalizer and Mobility Boots your laning phase is pretty much over. Your roaming becomes easier and faster. And the mid game is all about roaming and skirmishes(small teamfights, when there is not including all allies and enemies). Talon absolutely shines in that stage of the game. He becomes monster with few items and not so easily killed. At that point basically Talon's job is to push lanes fast, try look for lonely enemies or attend those skirmishes. Enemies will afraid of your damage. Winning those small fights will give your team lead and objectives.

Most of time I get Oracle's Lens so enemies won't know where I'm. That's the thing! Talon must not be seen to the enemy team. Don't be afraid entering enemy jungle if you are fed. Just start preying and killing that poor marksman or anybody squishy enemy who were farming in their own jungle. It's like a playing cat and mouse game.

Splitpush is not always a good choice. If you are fed you are really needed in teamfights. You can kill fed enemy carry and thus carrying game for your team. Splitpush if you see enemy can't really retaliate to that. I mean by not forcing teamfights or taking objectives.§

Late game

Late game is when all players start to gather and move together. When playing Talon you should reduce or stop roaming and start being close to your team. Teamfighting becomes very important in that point of the game and usually decides the games. Sometimes if I really feel that I can win the game by catching someone in enemy jungle, I get Farsight Orb. And also it may save your team when using on non-warded bushes. There are sometimes stupid players who goes to farm or push alone. If you see enemy doing then go after him/her.

Teamfight

Teamfighting is not really simple. As Talon you need to kill that carry in the enemy team. I find easiest target marksman. Usually by end of the game they do tons of damage and if not killed they might carry whole teamfight. If enemy's mage is fed, then you have to focus him/her. Be aware that mages usually have Zhonya's Hourglass. In teamfights fed assassins are not good choice to focus unless they are in bad position and you see it as easy kill. Then do it. Why I don't like to target assassins? Because many have some kind of variation of mobility to escape your burst. And you don't wanna chase them in the middle of enemy team.
So briefly about concept of teamfighting. The main targets are Marksmans and Mages. In order to kill your target fast and with no retaliation of enemy team, you have to know your position. So when teamfight breaks up Talon should not be in front line. Otherwise it usually will cost your life. As an Assassin you need appear last when all crowd control is used. That how exactly you play assassins.
There is always another way. The tactic I like to use is flanking. Talon is very mobile with Mobility Boots and Youmuu's Ghostblade therefore he can flank well the enemies. The idea of flanking is to strike from the sides of the teamfight. To use it properly you need to have Sweeping Lens and enemy team must have vision on you before you strike. When teamfight explodes you appear from the sides and kinda surround the enemy team. This will confuse enemy team because probably most of their crowd control are used. Usually their marksman or mage stays in the backline and will be quite easy to kill as they won't have anybody to protect them and with your burst you kill them in second. Afterward you will just walk away like a BOSS.

Like with any other champion, you need to practise long in order to master certain champion. There are many champions out there who needs more recognition. Talon haven't been popular because he is considered bad at Late game. If you read my guide, you see he is definitely not. There are some team compositions who completely dominates besides Talon even other assassins. Whenever I play Draft games. Solo or team ranked. I think twice before I pick. First check what team enemy have then try to counter it, unless you are first pick. I's still sure that Talon is often a good pick.

Thanks for reading! :D