In dom2 I had a surplus of astral gems one time as ermor and conjured up the eater of the dead. You can't summon another one while one is already in play, your summon just does nothing, like if you try to summon an ice devil, elemental king or whatever and they are all in play.
But, you could have one copy of each of the three versions of the eater of the dead. I don't remember what happens if one of them "evolves" into a version that already exists. I think he cannot evolve until the other one has evolved too. I don't remember.
Anyway, by the time you can actually summon one of these guys, and go to the trouble of feeding it, it's really not that threatening on its own. It does however make an excellent meat shield for a large army, as long as it's got help.
For extra fun, try wishing for an "unfettered"
