How about a change to how Karthus's E's passive works?

Something simple, his current E passive is pretty good, but I feel it could be a little better.

My idea is: when Karthus kills a large monster or gets an assist, he gets 5% of his max mana back. When he kills a champion or epic monster, he gets 10% of his max mana back. I feel this could reinforce his identity as a team deathmatch/ objective taking king :P It would also bring him back to the jungle without making his other roles OP

Note: this would be tacked onto his current E passive.