ToME: the Tales of Maj'Eyal

I dont know how i do that.Im just find group of this mobs and start casting some spells and all like that mob maked unkillable,but i kill them with Necromancer spell "Animus Purge" and got one in my summons.I really dont know bug this or game mechanic.Here's screenshots of this unit.
https://pp.userapi.com/c846123/v8461231 ... -XdLbI.jpg
https://pp.userapi.com/c846123/v8461231 ... 9OfqoY.jpg