Welcome to my Karthus guide :D

Karthus is a mage deathsinger who relies on skill shots and to finish enemy's, he is also one of the best 1 vs 1 champs ;) IMPORTANT: he is mid champ, but very bad at lane so you should always play mid Karthus...

First, lets introduce you with his abilities:

Death Defied - one of the best passive's in the game, especially in a teamfight when they kill you first (very rare), also good for pvp's

Lay Waste - your nuke spell, you should spam it on champion's (if you are better then them) or minion's (if you can't beat champ's 1 vs 1 so you need to farm some money)

Wall of Pain - good for chasing enemy's also reduces their magic resist so when you put the wall just spam Lay Waste on them

Defile - your close combat spell, you should turn it off as soon as the enemy gets out of the range cause this spell drains your mana big time...

Requiem - great spell to finish your enemy's, here are some hints about it:
1.NEVER,BUT NEVER use it in close combat or pvp unless your enemy's escape
2.Use it only when you are sure that you can kill the enemy
3.If your enemy escapes you should first go to any friendly turret,be sure that you turned off Defile and then you should use your ultimate
4. you should type at start of every game to your teammates to call your ulti when they need it, when they do so, first check that enemy, dont believe them too much...

For runes I use:

I use 9/0/21 masteries taking Archaic Knowledge in offense and all major mana regen masteries in utility: Expanded Mind Perseverence Meditation , also I take improved Haste ... this choice is simple, as I said before, Karthus thirsts for mana and that's why you need big mana regen, because of that you should use mana regen seals,masteries and great items like Archangel's Staff

Explaining items:

Archangel's Staff - your must have item... great at late game

Sorcerer's Shoes - not much to explain, standard caster boots

Banshee's Veil - great item versus casters, especially Malzahar...

Rod of Ages - gives enough mana and health. Also provides some AP, good passive

Void Staff - great for tanks, provides AP

Rabadon's Deathcap - must have late game item, OP passive

And now when we look at the stats:

2745 HP at lvl 18
3500 mana at lvl 18
128 magic resist
663 AP WITHOUT Archangel's Staff 's passive

looks like a Caster/Tank doesn't it but build is very expensive and I had it only once...

Not much explaining...

Max Lay Waste , take Requiem whenever you can. After Lay Waste max Defile and last max Wall of Pain . I dont reccomend any other skill sequence cause this is the proper one and you will fail with any other...

For summoner spells I use Ghost and Ignite but you can also use these: Clarity , Flash , Teleport or Cleanse

bad choices: Clairvoyance - well, usually supporters take it so DONT!!!

Exhaust - you have your Wall of Pain for slow and this spell is unnecesery

Rally - really bad choice for casters...

Pros: -Great at harrasing minions and champions (farming)
-One of the best and most efficient farmers in the game
-One of the best non-ult AoE CC abilities in the game
-High damage output in team fights
-Fun, active gameplay that rewards good positioning, placement and timing

Cons: -Must be very smart at consuming mana, especially for Defile
-needs good mouse clicking
-really bad vs ranged carries like Caitlyn

Your main farming spell is Lay Waste SPAM IT ON THE MINIONS cause it costs low mana but good for last hitting...

In teamfight your task is to harras their carry's wheter caster or AD carry's but also kill the supporter's. When the teamfight start's you should turn on Defile and start harrasing their carries and supporters with Lay Waste ...
if they kill you before teamfight ends you should use Requiem to help finish enemy's

This was my Karthus guide as I like to play him, be sure that he will not dissapoint you and if you have any ideas what else could I put in my guide pls comment and rate if you liked it :D Hope to see you in game ^^