Nightkin

Nightkin are creatures in Fallout, Fallout 2, Fallout: Brotherhood of Steel, and

Fallout/Fallout 2
Nightkin can be found guarding 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. Many of the Nightkin are equipped with cloaking devices, such as Stealth Boys. The only gaming effect of this is to give them a slightly ethereal look, as it does not affect the player character's chance of hitting them. After the Master's fall, many surviving Unity Nightkin scattered to other parts of the wasteland.

Fallout: Brotherhood of Steel
After the death of the Master, parts of the super mutant army gathered and formed a new faction, led by Attis. Many years after their regrouping, these mutants captured the Vault-Tec Corporate Vault in order to use the FEV reserves stored inside. Attis' goal was to continue the Master's plan. His army had an elite unit of Nightkin, much improved in skill, and equipped with high-tech stealth suits to enhance their abilities. These suits served the same function as Stealth Boys.

Fallout: New Vegas
Nightkin suffer schizophrenia due to Stealth Boy use. Nightkin follow either with Tabitha, Marcus or Davison. Many wear hoods and stop signs strapped to their forearms as makeshift shields. They use rebar clubs, miniguns, incinerators, super sledges or, in the case of Nightkin masters, bumper swords. There are some scattered elsewhere in the wasteland, in places such as Black Rock Cave.

A Nightkin camping on the outskirts of Novac kills one of the McBride's Brahmin every night around midnight. They can be found during the day, cloaked behind a boulder across the road and to the west of town. Here, they can be heard complaining about insomnia and trying to silence noises in their head that they believe come from the Brahmin. Some talk about receiving commands and advice from Antler, an inanimate Brahmin skull located in the basement of the REPCONN test site. The companion, Lily, is a Nightkin and a former soldier in the Master's army.

The Courier is sent by Jason Bright, the founder of the Bright Brotherhood, to take care of "demons" that are infesting the basement level of the REPCONN rocket testing facility. At Brooks tumbleweed ranch, a Nightkin will offer to sell the Courier a wind-brahmin for all their caps. If the offer is declined, they will turn hostile and proceed to attack. Another Nightkin, Dog and his other identity God, appears at the Sierra Madre.

Appearances
Nightkin appear in Fallout, Fallout 2, Fallout: Brotherhood of Steel, and

Behind the scenes

 * Originally, if the Courier accepted the wind-brahmin offer without asking about the price, no caps were removed, and the Nightkin gave a free Stealth Boy instead. With the game patch as of July 7–8, 2011, the Nightkin no longer give a Stealth Boy upon agreeing without question and will take all caps. The Nightkin will only have one Stealth Boy that can be pickpocketed or looted.
 * Joshua Sawyer conceived of the Nightkin becoming schizophrenic from Stealth Boys and was directly inspired by the novel The Invisible Man and his progression into insanity by taking an invisibility drug.

Bugs

 * In Fallout, some Nightkin that fight unarmed disappear (become "invisible") during the player's turn in combat and can not be targeted. The only way to end a battle with them is to either flee the area or have them be killed by an accidental fire, e.g. by other enemies that miss and hit them or using a burst mode of a weapon targeting any other available target with the intent to hit the supposed "invisible" Nightkin.
 * In Fallout: New Vegas, the shaders that make Nightkin invisible are sometimes bugged, causing them to remain invisible after deactivating their Stealth Boys (although they will be treated as visible for the purpose of calculating V.A.T.S. percentages). This has been observed in the REPCONN test site basement and during the encounter near McBride house in Novac, and appears to be linked to reloading a previous save during, or immediately after, an encounter with a nightkin (possibly after being killed).
 * After the Nightkin that has been attacking Dusty McBride's cattle is killed, a Nightkin master can appear near the tent in the middle of Novac; it can be encountered as far away as the Highway 95 Viper's encampment, and rarely can even spawn inside the McBrides' house. This Nightkin master will be neutral unless attacked by the player character but could be encountered fighting Vipers. If killed, this nightkin will have a minigun in very poor condition and possess The Screams of Brahmin note.