so this is my first build :)

i like to play karthus coz he does a huge amount of damage and is exceptionel in team fights.

if you play karthus be sure to be able to handle all the flaming from the enemy team.

but be aware that if you fail with karthus you will not just "fail" but fail miserably

Mark: Magic Pen. for obvious reasons. Karthus is a spellcaster and need to penetrate the magic resist the opponents will have/get.

Seal: AP. Karthus needs both AP and CDR, and doing some calculations it shows that its more effective to have AP seals and CDR glyphs.

Glyph: CDR. CDR i important for Karthus since it lets him use his Lay Waste alot faster, and more times while defying death. it is ofcourse also very useful for his ulti.

For masteries i'm going pretty basic spellcasting with the little "difference" that i max the "less time spendt dead". i feel that it is very useful for karthus to be alive fast, since you dont know when your ulti could come in handy.

for items i start of with boots and 1 HP and 2 MP.
i will then go for Mejais if it looks good or even okay when i can afford it (Karthus gets lots of kills because of his ulti) if the game isnt so good i'm going for "Will of the ancients".
i'll then go for the magic pen. boots and build towards zhonyas, always trying to get the "needlessly large rod" first if pos. (zhonyas is a very useful item for Karthus since he will almost always get focused, and with his "E" on and then zhonyas, they will regret jumping you).
then i'll get Rabadons or a situational item like, "abyssal scepter" for heavy AP team. "Void staff" for strong MR team or "Lich bane" if i'm the main pusher.
after that its time for "Archangels Staff" and "morrelo's Evil Tome" if the game made me buy "Will of the ancients"

"Lay Waste" is your harras skill and main damage. it is quick to set at give good damage if you can time it right. try to hit the enemy champ without hitting minions for maximized damage.

i tend to take "wall of pain" as second skill because if you use it well on a champion with no minions near it can either kill them or for them to recall with good use of the "Q"

"defile" is a very good skill to keep you a little longer in the fight with the mana, and later do a huge amount of damage

"Requim", or the, "omfg i hate you so much karth" does alot of damage but still requires a little technice. first of all be sure you cannot be stunned/silenced/killed while using it. be observant. a good idea is to ask your teammates to call for your ulti when they have an enemy low enough.

my favorite way to play karthus is to run right behind the tank in team fights. almost everytime the will focus you. push your "E" and use some "Q"s till you have them all together, then use your "W" and your zhonyas, with the "E" still active. when zhonyas is over, if the team isnt done or have run away, you will probably die, but dont worry, use your "Q" a couple of times and if its necesarry use your "R". DONT JUST USE IT WHEN EVER YOU DIE!!!

I take teleport, because it is very useful when going mid

the main reason for clarity is to save it till your teammates needs an ulti and you dont have enough mana. then just go a little back from your opponents, hits the clarity then hit your devestating ulti.

so this is basicly how i play karthus. please try the build, comment and rate it :)