Wraith

The Wraith is a monster in Doom 3 and the expansion pack. Wraiths crawl on the ground, can turn invisible and attack the player with scythe-like hands. It can be easily mistakken for an Imp while it's being summoned. Unlike an Imp, a Wraith can crouch

In the Making of Doom 3 book, id wanted the Wraith to form out of a cloud of insects, to hide in bodies and such, but it was scrapped due to the player wouldn't even be able to tell what the cloud was in the dark environment.