Phased

Creatures with the Astral tag are permanently phased. These include:

Phased creatures are out of phase and can only be interacted with by things that are out of phase or omniphase.

Phasing back in while inside a wall surrounded by 8 other solid walls will instantly kill the player. This also grants the achievement The Laws of Physics Are Mere Suggestions, Vol. 1 if this happens to the player.