User blog comment:Doeniel/Karthus, the Deathsinger Rework Ideas & Discussion/@comment-24385282-20160122232953

If you're going to give infinite mana, you should probably slightly reduce his base mana and heavily reduce the scaling. ~350~750 sounds better. You're almost definitely gonna have 100 cs by the team you reach level 18, so that's 200 bonus mana, plus mana from items and champion kills.

However, I do like the concept of gaining infinite mana on Karthus, and the opportunity for to steal Baron Nashor sounds great. The change to doesn't look too bad, either.