Hey all, this is my guide to karthus, this is not for noob's, this is only for ppl who know how to last hit decently.
Also u should try to duo q so your friend can get a soraka.

This work's perfectly untill 1800 elo.

Ye u need some kind of armor incase they trie to gank u. Also if u gank the enemy so u won't die if the minions hit u. In team fights, just go in them, use your e and put it off at time.

Just like i told, u need to know how to last hit. If u dont then this guide will suck for u. U will get focused 100% of the time so this build is perfect to damage as much as possible players around u.

Pro's = Karthus ulti is just so much fun :).

Con's= try to get a soraka in your team (in ranked) or else the enemy will pick soraka to counter u.

Tell your team they need to tell u if the enemy is low.

If u gank somebody, try to wall them (yes i know sounds stupid, but this lower their enemy magic resist) if they try to gank u, u use your e (which scare em off) and wall so u can countergank them.

The last hitting gives u mana in start. That's why a decent karthus doesnt start with a mana regen item.

Also you can engange perfectly, as long as your team is following u.