I became invincible and invisible to enemies due to a bug. I was running past a bunch of enemies to get to my ghost ( seldom works out well) in the Rhogar's realm part.
I ran down the stairs near where the first weird dragon-like creature and a forger are and attempted to open the first small gate you come to. I was getting attacked by multiple enemies but when the gate opening animation triggered they suddenly left.
I thought I had just gotten lucky and seeing my health bar was empty, I drank a potion. My health bar remained empty. So I drank another. Still empty.
At this point I suspected I must have hit a bug so I walked back out of the small room. None of the enemies responded to me but I was still able to attack them. Clearly as far as the game was concerned I was dead.
I also discovered that I could no longer interact with objects. No prompt showed on shards. I didn't try leaving an area or interacting with any other doors but I suspect the same would apply.
Unfortunately I discovered that falling, even from a small height killed me. Thinking about it that makes sense since it's not like the ground needs to detect you.
I suspect that any door would work for this but the chances of timing it so an enemy drains your last health just as the door animation triggers seem pretty low!