Some monsters in Heretic (Golems and Undead Warriors) can appear in a form of a ghost. These have the GHOST thing flag, which makes them translucent and not hittable by certain attacks, which are defined as non-magical. Projectiles that cannot hit ghosts will simply travel through them as if nothing was there.
Additionally, the Disciple of D'Sparil seems to possess the ability to flicker in and out of tangibility when preparing its attack, so a player might find a couple of their crossbow bolts going right through them.
Other bad guys will aim erratically at a ghost bad guy, in much the same way they do at a player using a shadowsphere, if they are infighting with one.
Ghosts cannot be hit by any attacks which have the THRUGHOST flag, including:
- Staff - Either mode
- Ethereal Crossbow - Small side bolts
- Phoenix Rod - Normal fireball (however, splash damage will harm them)
- Firemace - Either mode
- Undead Warrior's green and red throwing axes
They can be harmed by other attacks, such as: