Many of the nightkin are equipped with cloaking devices, such as Stealth Boys. They are superior to other super mutants in skill, equipment, and weaponry. After the Master's fall, many surviving Unity super mutants had become a chaotic wasteland-wandering force, set on destroying everything it encountered.
Nightkin can be found in the upper levels of the Cathedral, as well as inside the Mariposa Military Base, and in random patrol encounters in the cells around the base.