Hi I'm Armored Chaos and this is my first guide. I belive Karthus is one of the best AP carries due to his constant damage and global ult. This build will focus on a tanky karthus who can also deal tons of damage.

I take MPen marks, mana regen seals, cooldown glyphs, and movement speed quints. Other options could be armor seals, AP glyphs, and AP or flat hp quints.

Standard AP masteries 9 in offense tree for magic pen and 21 in utility tree for cooldowns on summoner spells and increased buff duration for your blue buff.

I like to start out with mana crystal + 2 hp pots. Other viable options include stacking 2 dorans early or boots + 3 hp pots. I like to rush tear of goddess to solve my mana problems and then get sorc boots. I then get catalyst for sustain and durability and then rush rabadons. After rabadons I would upgrade the catalyst to a rod of ages and then upgrade the tear to a archangels staff. Depending on how the other team is set up I would get zhonyas hourglass and banshees veil as my defensive items.

I get 2 points in q early for farming and a point in wall at level 4. After that I level up my ult whenever I can and then level up lay waste, defile, and wall in that order.

I take exhaust and flash. Flash lets me chase and escape from ganks easier. Exhaust is used for 1v1ing in lane phase and also can be used for chasing and running away.

Make sure in team fights you try to land your wall on the enemy carries and defile them. Try to land lay waste on 1 target at a time for double damage.

Don't forget to snipe low health targets with your ult.

Remember to farm well early

Please leave constructive comments. This is my first guide and I will be improving it based on feedback! Thanks