I'm curious, the decaying ability (healing kills u) + retribution + necromancy.[...]
Now I hope I would be completely wrong about that, but if not it would be hilarious.
Sorry but you are wrong.
Necromancy goes through decaying you are healed normally even with decay so you don't need to try and think about crazy interaction yet.
That is actually one of the only heal available (unless you are undead) when you are under the effect of decaying.
