With this Karthus build you will reach your complete build in under 50 minutes or so if you are sufficiently farming minions and killing champions. For your last item I recommend Abyssal Scepter most of the time, however if the majority of your enemies are stacking magic resist, Void Staff will be a better option. Once an enemy gets above 58.55 Magic Resist Void Staff will give a greater penetration than Abyssal Scepter does.
It is also important to remember that Abyssal Scepter will only affect enemies within 1000 range (the same distance as a vision ward) this is just over double your Defile range, therefore will not affect running enemies when you use Requiem.

Build 2 for Dominion