I agree. It's immersion breaking thing. I can't understand concept of it. Phisically immune creature, that can cause phisical damage?
Ok. maybe it's kind of emanation of "mental force of caster" but why in that case it stays alive after caster dies? It's contrary to ingame lore, btw. And contrary to behavior of it;s prototypes which disappeared after their "caster" died. I'll cut here to avoid spoilers.
I'm not talking about game mechanic here. After all it's typical part of any magic system, be it this game and other CRPG.