What do you think of this Karthus change?
If Karthus gets 2 or more kills with his ult (the ult kills the targets, not from a DOT or the environment like Baron) when he is in his passive, he comes back to life (revives).
His revived health and mana is that of the collective health his killed targets were at when his ult hit and killed them. So if 1 champion died from his ult dealing the remaining 250 of their hp and 1 with 150, he would revive with 400 hp and mana.