Partial invisibility

From Doom Wiki

Partial invisibility is a visual effect which affects certain characters in Doom and its progeny. The thing in question retains its shape, but instead of color the renderer draws black pixels scattered randomly (applied by colormapping the pixels behind it to darker shades), which makes the object more difficult to see. Although the engine theoretically supports partial invisibility for any actor, in vanilla Doom it is allowed only to the Spectre, and to a player under the influence of a blur artifact.

In Heretic, Hexen, and some source ports, partial invisibility is achieved by translucency (using the TINTTAB color map). Heretic and Hexen thus feature several "ghost" monsters as well as invisibility pickups (see Shadowsphere, Icon of the Defender).