Hello, I'm XxXyolomasterXxX. I have recently been toying around with gp10 and found that it works well with Karthus and his afk farm at turret laning phase. With this build you will be extremely weak and prone to ganks early, but a mid-late game monster. Since you aren't even really in lane you can pick him against everything, just remember to push when the enemy mid roams so they think twice about doing it again.

The runes are here to help you scale. Your quints are for easier q last hitting in lane. Pretty simple.

Since you aren't rocking flash you're taking that ms mastery at the end, also take gp10 for your scaling and take summoner cooldown because revive cooldown is op.

Simple Karthus build. You build the same as any other build pretty much just grabbing different starting items. Late game you have around 700-800 ap, which with void staff is a 1k damage nuke to anybody with little to no mr (R.I.P. carry).

Easier afk farming. You have a ton of mana regen so taking an extra early e point for mana regain on kill isn't essential.

Revive teleport is amazing for Karthus. You essentially have 4 lives including passive, and this essentially forces a 6v5 late game baron fight. No way you lose that one. Always remember to ult on your second death, and never the first.

What you do all game pretty much. If you can't farm with Karthus don't play him.

Overall this Karthus is very strong with a late game team fight comp, but it can fit into everything. I highly recommend trying this. It is true freelo. When playing this build I recommend listening to this song on repeat: http://www.youtube.com/watch?v=eEoMI9BwHp4