Hello and welcome to my Karthus guide. I hope you like my guide and if you want you can post me a screenshot of your stats with this build.

So I've been playing LoL for about 6 months and I bought Karthus at about level 15 after my friend recomended it to me. All of his spells are great and he's in my opinion one of the best AP carrys in the game.

The runes I choose work out great for Karthus. The flat AP, Magic pen, and health really helps from early to late game.

Masteries are pretty standard for AP carrys getting points in ghost and flash plus increased buff duration and some well needed offensive penetration.

Archangel's Staff is a must have item on Karthus. It works so well with his Q wich is the most spammable spell in game. You'll get that 1000 extra mana in no time. I choose to get a Meki Pendant at level one then building it into a Tear of the Goddess so you can get some mana early on. Then I build either Sorcerer's Boots or Boots of Speed and a Mejai's Soulstealer. If you're getting fed early game I would recomend getting Mejai's ASAP.
Then I try to rush Rabadon's Deathcap. It will boost your AP to a huge level and your damage will go from poor to extreme. After that I usually build my Tear into Archangel's Staff, but if you're getting ganked alot you might wanna go for some extra health and get a Catalyst for Rod of Ages as it is in the build anyways. After you got your 2 real core items (Archangel's and Deathcap) you should easily dominate your mid lane. If you haven't killed their mid tower yet do it ASAP and go push the lane that needs most help. Global gold win you games right (:.
Getting some towers down and some dragons will help you get your next item: Rylai's Crystal Scepter. Fantastic item for Karthus as your Defile will slow anyone in it. The health is awesome to. This is really going to help you survive. If you got all of these items it's pretty much GG. Unless the other team starts stacking magic resist then all you have to do is: GET A VOID STAFF! This will basicly whipe out all of their cash spent on magic resist and you'll be raping their faces anyways.

Or you could go with that 5 archangel's and deathcap build wich suprisingly many do. It's alot of fun (:

A Guardian Angel isn't a bad choice when playing Karthus. It really helps out with your survivability.
Banshees Veil is another very good choice for Karthus. The passive block is amazing.

Requiem > Lay Waste > Defile > Wall of Pain

His Lay Waste does so much damage and you really want to max it out first. I get defile at level 2 to help with my mana pool as I'm last hitting, getting another point in Lay Waste at level 3 and getting my Wall of Pain at level 4. At level 6 you'll have a pretty much guaranteed kill.

His passive is also fantastic in teamfights and if you get focused and die most people are stupid enough and just stand in your defile.

Ghost

Karthus is a slow champion and ghost really helps with that.

Flash

Karthus is easily ganked and flash is the best escape tool in game.

Teleport

If you want to farm alot and loose your tower really late you want to get teleport. I choose it sometimes when I'm doing premades with mates that can just put wards in a bush and I just come and gank them from nowhere.

Ignite

Not the best choice for Karthus as you can just finish your opponents off with your ultimate but I can see it being used.

Clarity

I have actually never used this on Karthus but I see alot of Karthus players who skip Archangel's and roll with this instead.

I made this build for ranked mainly as it gives you alot of survivability and that's always good in ranked games.