The Nightkin are elite and unusually intelligent, stealthy, and vicious Super Mutants, who originally served as the elite of elite within the Master's Army.
Today they are rarely encountered and many are suffering from over exposure to Stealth-Boy technology, rendering them insane. There are select groups of Nightkin still operating intelligently and it has been theorized that these extremely dangerous groups may have developed an immunity to the Stealth-Boy side effects.
In my personal future lore of the Fallout universe, the United States slowly forms several rudimentary "nations" and one is a resurgent Super Mutant force in the north. Having salvaged and finished the research to create self-sufficient breeding Super Mutants (as laid out in Fallout Tactics) these mutants have been slowly rebuilding.
In terms of modelling, I enjoyed swapping some arms, adding a Laser Rifle (from Brother Vini) and painting some albino Mutant Hounds (presumably they are equipped with Stealth-Boys on occasion).
I attempted two different schemes, a blue as seen in New Vegas and a paler purple as depicted (somewhat) in Fallout 1. Most people prefer the blue, and I must admit, it's contrast probably makes it the superior choice!