Ghosts (Heretic)

Some monsters in Heretic (golems and undead warriors) can appear in a form of a ghost. They are translucent and unable to be hit by certain attacks which are defined as non-magical. Projectiles that cannot hit ghosts will simply travel through them as if nothing was there.

The Shadowsphere temporarily confers all the properties of being a ghost to the player when used. Monsters' aim becomes erratic when attacking ghosts, whether the targets were players with the Shadowsphere or ghost enemies engaged in infighting.

Additionally, the disciple of D'Sparil possesses 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.

Immunities and vulnerabilities
Ghosts cannot be hit by attacks which have the 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
 * Iron lich's ice ball (but the shards can hit them after it explodes)

They can be harmed by other attacks, such as:
 * Gauntlets of the Necromancer
 * Elven Wand
 * Ethereal Crossbow - Center bolt(s)
 * Dragon Claw
 * Hellstaff
 * Phoenix Rod - Powered-up attack
 * Timebomb of the Ancients
 * Most monster attacks, including undead warrior's melee strikes.

Iron lich tornadoes do not home in on ghosts.

Trivia

 * The Heretic manual marks weredragons as one of the creatures to have a ghostly version, however, this is false.
 * Another type of ethereal monster appears in Hexen, but only during the final battle with Korax. These are known as mash monsters, and lack any of the extra protections granted to Heretic's ghosts.

Technical
Originally, Heretic used the thing flag, recycled from Doom's spectre and blur artifact effect, to confer both translucency and weapon immunity to an entity. However, some modding-oriented ports such as the ZDoom family have decoupled translucency from the ghost immunities, and use a flag to signify that the latter effects should be applied.