UPDATED AFTER NAUTILUS PATCH (v1.0.0.134)

Changed Mastery points and mastery section.

UPDATED AFTER LULU PATCH (v1.0.0.136)

Added new build with different runes and item build.

Dear ******s,

I realize now that i am being downvoted on this guide because there is "no escape summoner spell." I then laughed for an extensive period of time and realized that the downvotes are coming from people that do not know how to properly play Karthus. Karthus is not a champion that cares about escapes. He is more often than not initiating fights with Wall of Pain or going straight into their team with Defile up. So before more of trolls downvote this guide, it would be very kind of you to actually learn how Karthus is supposed to be played before the downvote. Thanks.

Sincerely, The Author

I assure you this is not a troll build. Nor is it a way to troll a team. It is a way to carry your team. And carry hard. We all hate Karthus' ultimate and hate trying to kill him. But let's put him in the jungle where he can get a consistent source of farm and experience. This is a tactic that only Soraka can counter. So let's start talking in-depth about the Zombie Jungler Karthus.

PROS

One of the best AP carries in the game
That ult
Magic and armor shred
Constant AoE Damage
'Dat ult
Never dies
Did i mention his ultimate?

CONS

Incredibly squishy
Dangerous jungle

Death Defied
This is your passive. This is why Karthus never really dies and why you can run into their team and not care about being focused. You still have 7 seconds after being slain to cast more spells and your ultimate. Rolling with Revive? Come right back up and Requiem their team for a PENTAKILL.

Lay Waste
Your main jungling ability and damage dealer throughout the game. This ability is rather hard to aim and takes some getting used to. It will be aimed at where ever your cursor is hovering over at the time you click it. Remember this, it is important.

Wall of Pain
I want to call this the best slow in the game. Nunu & Willump's slow ball slows for a long duration and hard, but it does not shred armor and magic resist. Wither is cool, it isn't AoE, and it doesn't stay there blocking enemies from an area. I dare you to find a better slow than this.

Defile
Dare your enemies to come close to you while you have this active. Especially with Will of the Ancients. They will only hurt themselves and heal you. This is an important jungling ability, too. But it burns mana. So just have it up so it deals damage for 1 tick, then turn it off and keep doing that.

Requiem
This is why everyone hates Karthus. A global ult that can deal damage from anywhere on the map. Nothing more needs to be said here. Press it to win.

Prioritizing Ranking
Requiem -> Lay Waste -> Defile -> Wall of Pain

Marks
Greater Mark of Magic Penetration - Magic penetration. You want your abilities to hit as hard as possible. Shred as much magic resist as possible. Go for gold.

Seals
Greater Seal of Ability Power - Ability power. Flat ability power, to be more specific. You need as much ability power as possible at the early levels to jungle and not die as well as jungle faster.

Glyphs
Greater Glyph of Ability Power - Again, flat ability power. Go big or go home.

Quints
Greater Quintessence of Ability Power - Flat ability power again. Damn, i'm creative.

Important Masteries For Zombie Jungle

Summoner's Resolve
You need the extra gold you're going to get from the improved Smite. And i wish it reduced the cooldown on Revive too, but it doesn't unfortunately. Nonetheless, the improved Smite is what you're after.

Arcane Knowledge
So you can deal some real damage when you gank or ult or just team fight in general. Pretty standard on any caster.

Executioner
To get that extra bit of damage to get that kill with your ult or any damage you deal. It's good for the killings of the enemies and such.

Runic Affinity
You use mana taking out the jungle. Keep blue buff on you as long as possible so you can farm more.

Good Summoner Spells for Zombie Jungle

Smite - You are jungling, it is a necessity to jungle.
Revive - I NEVER DIE. You spent 10 seconds getting me to "die" after which i was still dealing huge damage for 7, and then instantly respawned and ulted your team for a pentakill.
Flash - Always viable, always usefull.
Teleport - More so for just zombie Karthus. You would pair it with Revive.
Ignite - All casters can use Ignite to maximize damage.
Exhaust - Only if you roll with improved Exhaust for the lowered magic resist and armor.

Okay Summoner Spells For Zombie Jungle

Clarity - You do use quite a lot of mana using Defile and Lay Waste in teamfights, but you should be the priority for blue buff anyways.
Ghost - Not really necessary but sometimes helpful to catch up to faster champs.

Trolling Summoner Spells for Zombie Jungle

Heal - You are not concerned with dying.
Clairvoyance - Your ult doesn't need to be aimed to hit all the enemies.
Promote - LOLOLOLOL
Cleanse - Not concerned at all about dying. You want them to kill you.

I am going to list the items in the order of which way to buy them in game.

& Cloth Armor and 5 Health Potions
You are going to need these to start out jungling. Sell the armor as soon as you grab the first Amplifying Tome.

Sorcerer's Shoes
You are an AP caster. Get them so your abilities hit harder

Hextech Revolver
This will not only make jungling easier, but it will make you harder to kill. Considering that you deal all AoE damage, you are going to heal for a lot with this.

Mejai's Soulstealer
This is a core item. I don't care how you're doing in the game. Get it. You will get stacks during the 7 seconds of Death Defied and more when you use Revive. Become the target, and punish them for killing you.

Rod of Ages
For more health and mana and ability power. Don't build anything else for health on him, though. It's not necessary. And a waste.

Rabadon's Deathcap
More AP, with more AP. I like where this is going.

Will of the Ancients
Go back and finish this after you have Rabadon's Deathcap. You are hindering your build if you get this before Rod of Ages and Rabadon's Deathcap. With this, you are going to help anyone who deals magic damage on your team sustain through a teamfight. Double win if they have it too and you get the double buff.

Zhonya's Hourglass
I find the last item to be preference. You could get Void Staff or Abyssal Mask for the last item to hit harder, but this has more AP and a nice active. But then again, we aren't concerned with dying and evading death as a zombie, are we? Or, if you have the money or just want MOAR DAMAGE by all means, get another Rabadon's Deathcap.

This is where playing Karthus becomes tricky. You are squishy because your runes don't have armor. Nonetheless, start at blue. You need a leash to do this. Make sure that when you start using Lay Waste that it hits all the monsters in the camp. It will make it faster and safer for you. Pay attention to where your cursor is, always. After clearing blue buff take a point in Defile and move to wolves. The jungle gets trickier now. Be sure that Lay Waste is hitting all the monsters, but you also have to make sure that you only turn Defile on for 1 tick, and then turn it off and keep doing that to jungle faster. And after that you do the same thing at all the jungle camps. Moving from blue -> wolves -> wraiths -> red -> double golems.

Ganking

Always, always inform your team members that you are coming to gank well before you do. They will be prepared and save their cooldowns to secure a kill. But what you want to do is drop Wall of Pain on top of your enemy. And then lead their escape path with Lay Waste and turn on Defile so they can't run away from you or towards you. If you didn't get the kill, but got them low, then just Requiem. You might even wind up getting an assist in an entirely different lane or kill their jungler.

Teamfights

You are the only AP caster that can throw caution into the wind and face tank their team. Especially with this build. Go rambo on them. Start Laying Waste all over their team while you Defile them and drop a nice big Wall of Pain on their face. When they're exhausted from all this forceful backdoor penetrating sing them a lullaby with Requiem.

(You like what i did there? I do too...)

We all know that Karthus is an amazing AP carry. But not everyone is used to seeing him in the jungle. A team can completely forget you are jungling or even present in the game. And then, all of a sudden, pentakill. All in all he is a fast jungler and his ganks are pretty strong too with Wall of Pain. Try this build out and tell me if you like it at all.