Mash monsters

"Mash" monsters are phantom versions of certain Hexen monsters that cannot be placed as things, but appear via scripted events. They are identical to the normal version of the monster except for two things: they are translucent, like Heretic's ghosts, and they vanish instantly when killed rather than having death animations. The term "mash" comes from the script spawn names.

In the original levels, mash monsters are produced only by scripts invoked by Korax in the second phase of battling him in the Dark Crucible. If Korax is slain quickly, they may never be seen in a playthrough.

Aside from possible flavor considerations, it is likely that the inclusion of mash versions of these monsters was intended to reduce the chances of hitting the visible sprite limit during the Korax battle by not leaving corpse sprites behind.