AP Karthus - by Cold Fusion

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This guide is one I've written for Karthus, whom I've taken a liking to in this past week. He is widely regarded as underpowered and weak by most of the community, but he can be one of the most dangerous champions if you play him right. Anyways, to the guide.

I. Skills

Karthus has some of the most interesting skills in the game. They are:
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Q - Lay Waste: Karthus unleashes a delayed blast at a location, dealing damage to nearby enemies. Creates a delayed blast at Karthus' cursor position. After 0.5 seconds, deals 35 / 55 / 75 / 95 / 115 (+0 per ability power) magic damage to each nearby enemy, this deals double damage if it hits only a single unit.

W - Wall of Pain: Karthus creates a passable screen of leeching energy. Any enemy units that walk through the screen have their move speed, armor, and magic resistance reduced for a period. Creates a 800 / 900 / 1,000 / 1,100 / 1,200 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.

E - Defile: Karthus passively steals energy from his victims, gaining mana on each kill. Alternatively, Karthas can surround himself in the souls of his prey, dealing damage to nearby enemies, but quickly draining his own mana. 
Passive: When Karthas kills a unit, he restores 20 / 27 / 34 / 41 / 48 mana.
Active: Defile drains Karthas' mana to deal 30 / 50 / 70 / 90 / 110 (+0 per ability power) damage to nearby enemies each second.

R - Requiem: After channeling for 3 seconds, Karthus deals damage all enemy champions. After channeling for 3 seconds, Karthus deals 250 / 400 / 550 (+1 per ability power) magic damage to all enemy champions (regardless of distance).

Passive - Death Defied: On Death, Karthus enters a spirit form where he can cast spells while dead.

My typical spell lineup is this:


Most people that play Karthus like to go Wall of Pain before they max out Defile, but the way I build Karthus gives him alot of mana, so he can use Defile at will. I use Wall of Pain for the slow at first, and don't rely on it's magic resistance reduction until later. This allows me to cast Lay Waste and slow with Wall of Pain without running out of mana early on.
An important part of this build is knowing how to last hit minions. When you kill a minion, Defile will give you some mana back, so if you're not last hitting with this build it's a waste.

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II. Summoner Skills

Heal and Teleport - Works really well. Since you have Defile to give you mana back, if you get Heal you will have to recall less. Teleport works well too because it helps you get back to the fight fast, important because Karthus needs to get kills early on.

Heal and Flash - Never tried this one, since Flash was put up at level 30, but in theorem it would work. You have a slow with Wall of Pain, but Flash is useful for getting in to get a couple extra hits in order to not use your ult, or escape, which is important since Karthus is so squishy.

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III. Item Build

Karthus' reccomended items work very well for Karthus, but you can do a bit of branching out, depending on what you're playing.

1. Doran's Ring - A good starter item. Some nice mana regen to help along with Defile, a bit of AP, and some Health to keep you from being too squishy early on.

2. Sorcerer's Shoes - Good AP with Movement Speed. Good because you can catch up to slowed enemies quickly, and keep enemies in your Defile circle.
2. Tear of the Goddess - You're going to be using Lay Waste alot, so this is a good way to get alot of quick mana.

3. If you got Tear of the Goddess, move up to Archangel's Staff. Very good item, and the 2% mana to AP is good later on. It's ok to get this item before the boots because you can use your Teleport (if you have it) to stay in the battle if you need to port out. If you started with boots, get Tear of the Goddess up to Archangel's Staff from here.

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4. Here's where you can change it up a bit. If you're playing a Tank heavy team, I like to go Glacial Shroud for the armor bonus / Cooldown Reduction. If you're playing a Caster heavy team, I would go for the Abyssal Scepter. If it's balanced, I like to go for Abyssal Scepter because of the AP and the enemy magic resist reduction aura.

5. Start building Zhonya's Ring. Usually you can't finish it time before the game ends, but I like to go one Blasting Wand and the Sage's Ring first, then get the other Blasting Wand up to Zhonya's Ring (if i have time.)

6. Typically the game will never pass the point you get Zhonya's Ring, but from here it is your choice. There's alot of options, but it's probably best to go anything with AP (Deathfire Grasp, Lich Bane, Guinsoo's Rageblade, etc..)

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IV. Runes and Masteries

--Coming Soon--

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V. Other Champions

a. Who you have an advantage over:

Melee Champions, a few ranged heros.

Evelynn (if you have oracle's elixir / vision ward)

b. Who to watch out for:

Long ranged champions / dangerous burst / immune to slow / slows and stuns

Master Yi

c. What can go either way:

Steady damage heros / short ranged / support

Twisted Fate
Twitch (if you have oracle elixir / vision ward)

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In summary, Karthus can be extremely dangerous or horrible. It all depends on how you play him. You always have to remember, even though you are squishy, if you die, the fight isn't over because you can still cast spells for 10 seconds after death (and sometimes finish off the enemy with your ultimate). Karthus is great at last hitting minions, and can take care of alot of champions fairly easily. I would reccomend Karthus for a more advanced player, but new players can do well with him too.

Keep in mind this is my first guide, so if you have any suggestions on how to better it, please post. Thanks for reading, hope you got something out of it.