GamesRadar has cranked out a three-page "noob's guide" to the factions we'll be siding with or pissing off during our time out in the the Mojave wasteland. A quick look at the Nightkin:
History: The Forced Evolutionary Virus (FEV) program was designed to help humans cope more easily with the hardships of a post-nuclear wasteland. While it does do... that, those exposed to super-concentrated amounts of the virus transform into Super Mutants hulking, genderless brutes barely capable of authoring a coherent thought other than (Gaaaah! Kill!)
There are exceptions, however. In the early 22nd century, a severely mutated... uh... )man) known only as the (Master) began experimenting with highly concentrated quantities of FEV on human and other subjects. While the majority of the human experiments emerged as the slavering savages we've come to know and love, a few just a few - emerged with much of their original intelligence intact. These either became officers or Nightkin in The Master's army. The Nightkin were actually quite skilled in the art of ambush, often using stealthboys (active camouflage devices) to great effect.
When the Master was defeated in 2161, the super mutants dispersed, creating roving gangs throughout the Southwest. Before his utter destruction, however, the Master imprinted his attributes in several (talented) super mutants. To one called Attis, he gave his genius level intelligence. To Melchior, his Psionic abilities. And to Tabitha (leader of the remaining Nightkin), he gave his multiple personalities.
Role in Fallout New Vegas: One of the dangers of prolonged use of a stealthboy is heightened paranoia, delusions, and eventual schizophrenia. And Super Mutants were never known for being totally stable, no matter what intellect remained intact. So a Super Mutant who now suffers from paranoid delusions cannot be good for anyone.