Insubstantial
All insubstantial monsters are without physical form of any sort. They are immune to petrification, constriction, and Sticky Flame.
Players turn insubstantial while in Storm Form or Wisp Form. They aren't immune to constriction, but retain the other two resistances. (Note that Storm Form is immune to constriction on its own)
Insubstantial monsters have no special resistances or immunities to weapons of any sort, unlike, for example, NetHack's shades.
