Dont nerf Karthus, just revert dark harvest.
Im not gonna write a whole essay on this but Karthus was truly only broken because this IS his rune. Only 2 champs in lague even use it anyway. Brand and Karthus. "Lux" i hear you say, however aery is far more superior since her R isnt a 100% chance to hit. Just revert DH and give me my rune back :(