Well this is my first build, so I decided to do it on my best champion. I bought Karthus about the time Katarina came out (yea I've been playing that long). Karthus is my favourite because he doesn't require much ability power to be affective.

Karthus is a very squishy champ that needs sustain to do enough damage, so why not build tons of health. Like I build karma I build Karthus as kind of a soaker. You "soak" damage from your other carries and keep enemies away from them with wall of pain. Plus people have played with the Karthus that owns A.K.A me, so they tend to focus Karthus A LOT, so yet another reason for the health.

When using lay waste notice where the enemy tries to juke. If they run back (which is most frequently the case) you cast it behind the enemy. Also remember that your lay waste DOUBLES damage when it is a single target. Never be afraid of that Akali thats trying to intimidate you mid with this build you can easily 1v1 her.

Wall of pain is sometimes a PAIN in the *** because of how it corresponds to the way Karthus is facing; if you are chasing Kennen and you are going left and he goes up, or north you wall of pain will go in a line which he can easily get out of by going left or right, which at that point if he is low enough you requiem and kill him.

Your ulti is one of the most annoying for the other team. Ive gotten pentakills off of Karth's ulti but its all about timing. Sometimes your teammates will tell you the correct time for your ult then not ***** you out about ksing, but usually your not that lucky. Most of the time you have to look for it yourself. Monitor the other lanes when you can, and when you see someone low run off hit your R (out of the turret =P) and get the kill. Sometimes your lucky and you even get someone from top and get a double kill. But with requiem its all about timing.

Don't just not try the build and say it sucks because it doesnt look like a regular mages build because karth isnt a regular mage. Dont not try it because this is my first build either (remember I have been playing karthus FOREVER) and remember that karthus takes time to master.