User blog comment:HozzieHozzbourne/Strongest champion outside of the game/@comment-4915885-20130919212001/@comment-8506165-20130920064829

"with each soul consumed he becomes more powerful."

That'd be.

Karthus has no reference to eating souls. Closest thing is his old lore mentioning he maintained an army of the undead in his swamp, but the new lore doesn't mention anything like that (he doesn't even live there in his new lore).