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They have an attack which stuns the player, followed up by a grab attack which looks a lot like them trying to bite(?) your head across a few seconds, doing immense damage.

I think it's probably intended to be that, yes.

I haven’t played demon souls but if they’re like the other mindflayer equivalents that fromsoft has made for their other games (the lanterns from bloodborne come to mind or the Elden ring dlc insta kill madness guys) they absolutely do instantly kill players with some mind based shenanigans.

They stun, then skitter over and do a grab attack that sucks your brains, usually a one-hit kill as you encounter them early enough.