Spectres in Doom 64 are similar to their classic counterparts.
The Spectres of Doom 64 are resurrected carcasses of those from Doom 1, resurrected by the Mother Demon. They are made from systematically altered dead carnage back into corrupted tissue. The mutations were devastating. The demons have returned stronger and more vicious than before.
Great! Just what you needed. An invisible (nearly) monster. These beasts will eat your face off. Did you expect a walk in the park?
In Doom 64, inactive Spectres are initially rendered as opaque Demons with a green tint, becoming translucent upon detecting the player. Upon death, they revert to an opaque state again. Spectres also appear in the Doom 64 Cast of Characters sequence at the end of the game, unlike the Spectre in the Doom II cast sequence. They first appear in The Terraformer.
There are some tricks that can help make Spectres more visible: their shimmering outline is much easier to see with the inverted colors of an invulnerability sphere, for example, and they feature a fully visible blood-splatter effect when hit.
Despite the fact that Spectres have the effects of partial invisibility, their effect does not throw off the aim of other monsters when infighting.
In many OpenGL source ports, as well as in Doom 64 and the PlayStation version of Doom, Spectres do not "shimmer", but are instead rendered as near transparent. This is because the partial invisibility effect is very difficult to reproduce using such a renderer. EDGE, however, emulates the effect.