Karthus on Dominion is not such a good idea, but if you decide you like his skully face so much (like i do) and you still want to play him, than here is a build that will render you somewhat useful.

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Runes are standard for mages. AP en MP for max early game damage. Karthus has to rely on insane damage outputs, that will break down enemies before they even touch him.

Utility and MP ofcourse, penetration and more raw ability damage is the only sane thing to pick.

First you need to boost his damage, than you want to survive as long as possible to defend and keep enemies away from points long enough so your teammates can help. Karthus can defend a point on his own for quite a while if he builds enough damage to scare off enemies and enough armor / MR to survive a single enemy.

You will often get targeted, but this is okej. Use the hourglass and slow in combination with your wall to get out.

The wall is nice, but damage is nice, nicer, nicest. When skilling, try to max out Q before anything else, for in dominion farming is not an option, so the mana restore on kill from defile won`t be really beneficial. Besides, if enemies do get close, the defile is not going to kick in hard enough early game, to do good damage and they can escape defile easier after they kill you than they can escape from your well targeted Q`s.

Flash and ghost are my favourites for any type of champion on dominion, but promote or tower boost might work too. You will do best defending the mid or lower tower, so pushing lanes and boosting towers can be very nice.

- Karthus will not do well in a bad team. He will not break the enemy when you are in trouble, nor will he carry your team to the win.

+ Karthus will be one of the hardest to kill, mages when under a tower, will interupt people when they are capturing towers and as support damage, with his range, his slow from wall and ryjal`s scepter and with his after death spellcast, he can be a real pain in the *** to bring down and can definitly be the one thing making your team win.

Remember that most people like to take AD champions, so there will be a lot of people with low Magic resist and vulnerable to karthus Nuking capabilities. keep in mind that with a tanky ad, or another ad nuker karthus will win a 2 v 2, or even a 2 v 3 or more, when properly handled!

Upon dying, Karthus enters a spiritform, allowing him to continue casting spells while dead for 7 seconds.

- This is the best passive in the game: if you are placed right when you die you will be able to pick up everybody around you, or cast your ulti to finish those running enemies off. When you have about tons of ability power, you can do massive damage, even when you've been killed. When you get killed in a teamfight spam Lay Waste or try and finish off your enemies with Requiem. Requiem can also help your team to kill the rest of the enemy team, so you can use it to get assists, though it's funnier to get kills.

Lay Waste
Creates a delayed blast at Karthus' cursor position. After 0.5 seconds, deals 40 / 60 / 80 / 100 / 120 (+30% of ability power) magic damage to each nearby enemy, this deals double damage if it hits only a single unit.

Cooldown 1 seconds | Cost 20 / 28 / 36 / 44 / 52 mana | Range 875

- For Karthus, this is his main ability. This is your main farming spell, until you have enough mana to keep up Defile. You will want to max this first, as it is your best harrasing skill and you will want to keep enemies away from you. It. Be aware that enemies can avoid it, so try to predict where they will run to and cast it in front of the enemy.

- When you are dead, try to take away as much HP as possible with this. It can grant you assists or even some kills during a game.

Wall of Pain
Creates a 800 / 900 / 1000 / 1100 / 1200 distance wide wall through target point. Reduces enemy armor and magic resistance by 15 / 20 / 25 / 30 / 35 and reduces movement speed by up to 40 / 50 / 60 / 70 / 80%; debuff lasts 5 seconds. The wall lasts 7 seconds.

Cooldown 22 seconds | Cost 100 mana | Range 1000

- This is a funny and very usable skill. It is not even that hard to place, because of it's enormous range. You only want to choose this once in early game, for the extra slow the upgrades grant will not be better than the extra damage the other spells will give you. Use this skill wisely, for the cooldown is quite high and it WILL save you when a melee will come and jump you. Don`t cast it in front of a champion when he still has to come to you, for he can flash over it, wait or just avoid it someway, better let them come almost into range than cast it on yourself and than start dancing around with your enemy.

Defile
Toggle Off: When Karthus kills a unit, he restores 20 / 27 / 34 / 41 / 48 mana.

Toggle On: Drains 30 / 42 / 54 / 66 / 78 Mana to deal 30 / 50 / 70 / 90 / 110 (+25% of ability power) damage to nearby enemies each second.

- This skill will be a killer machine in late game, when you have 800-1000 ability power. It can also kills a wave of minions in the matter of seconds. Early game, you can't use it that much, due to your mana pool, which will be drained quite fast. When/if you die, you pop this until, you're actually dead. It does massive damage each second, which can get you some kills, when you also use Requiem. There's not much to say about it except to use it for farming and in teamfights - When you have Zhonya's Hourglass you can flash in, and use Defile + Zhonya's active. This will do nice damage for some seconds, before you will get targeted.
- Sometimes you simply have to die, in order to win a teamfight. Don't be angry, if you don't feel you have protected well enough, because the carry is more important in the end. The carry must finish them of, if you cant.

Ultimate: Reruim
After channeling for 3 seconds, Karthus deals 250 / 400 / 550 (+60% of ability power) magic damage to all enemy champions (regardless of distance).

Cooldown 180 / 150 / 120 seconds | Cost 200 / 300 / 400 mana | Range 10000

Cooldown 0.5 seconds | Range 550

Karthus can use this spell in many different ways. You can cast it to finish off enemies that flee, you can cast it mid fight to hit the enemy with massive damage, forcing them to flee and break, or just shift the balance of the fight. In addition, it can also be used to interupt in dominion, interupting enemies when they are capping a point. This can be useful if a friend is close and needs those extra seconds to help.