­Necropotence + Annihilator Dragon

Wow, one of the classic "Let's break magic for years" cards, and a dragon-eldrazi mash.

How about a dragon that brings stuff back? That might be fun.

And black has a nice firebreathing-alike ability, that fits well.

Awesome! Terrifying recursion if it hits you once, but only if it does get through.

Or if you have a Viridian Longbow in play. Normally, when someone says something like that, they're joking... but that's one heck of an activated ability...

Well, it doesn't let you recurse forever, because it locks the graveyards against new entrants. But yeah, bring back everything that's in there before you cast this. Mind you, the damage it deals means you won't actually need to bring back much stuff.

Overpowered for a black shivan? No idea what creature casting costs are nowadays.

Not overpowered, no. Might want to be legendary, or perhaps mythic (or both), but perfectly printable.

WHAT. Infinite black Regrowths for one mana? This is awfully dangerous, especially because black can easily sacrifice this if it ever needs to set up another one.

I would test the hell out of this before trying to release it in any format.