What does it do when used on an outsider, or eidolon? Does it flat-out fail, because these have no 'detachable' souls?
What happens if it is successfully used again a beastbonded witch sharing a body with her familiar? If Demmy only gets one soul, which one is it?
So many corner cases, so few answers...
I'd pretty much round in favor of the demilich kicking ass. Maybe not only does it get to eat an eidolon, but that summoner doesn't get to have an eidolon until they kill the demilich and set it free. Etc.