Nightkin

The Nightkin were presumably an elite warrior class in the Master's super mutant army. They can only be found in the upper levels Cathedral.

The Nightkin were all equipped with cloaking devices, the Stealth Boys. The only gaming effect of this is to give them a slight ethereal look, as it does not effect the PC's chance of hitting them.

The Nightkin in Fallout Games
The Nightkin appeared in the first Fallout game and only in the Cathedral. They also appear in Fallout: Brotherhood of Steel.