Karthus needs a hotfix nerf.
This is insanity. Throw Seraph's on a Level 1 Karthus and he's going to wreck half your team in 20 seconds max. The only possible way around it right now is to build for it late game going Dragons, stacking claws, or praying you can hit it with assassins because god forbid someone actually 2*'s him, it's over.
He doesn't even need a big nerf, half his ability damage that's it. It's still more than he had prior to 9.15.