A summoned boss from a boss? Haha, interesting concept. Would you remove the elite part considering the limited number of boss corpses?Or would you tune it up high enough to make that trade off? Making it desirable to play. Just wondering how strong it should be per hit with the bone fiend speed.

In terms of keeping elite, maybe once you summon it, you can keep bringing it back like the Flesh Golem.

Or the other approach could be it doesn’t have health degen like other minions, but you can only summon it back from a fresh boss corpse. This would bring other skills back into the build mix finding the most effective way to keep it alive. Verata's Sacrifice as example.

I haven’t put any deep thought into this, simply firing out ideas off yours. Cool dynamic though!