Attis

Attis is a super mutant general of a faction that remained after the destruction of the Master and the vats. He is a great enemy of Brotherhood of Steel. Attis is the main antagonist and the final boss of FO:BOS.

Background
Attis was a super mutant, one of the Master's soldiers, and after the death of the Master he became the general of the faction which tried to continue the Master's work, rebuild the army, and dominate Southern California once again. Attis is one of few super mutants with such a great strength, both intellectual and physical. He is tall, strong, and quite athletic for super mutant. He is also very fast and knows how to use all kinds of weapons. Highly intelligent and vindictive towards the Brotherhood of Steel, Attis took his group into hiding, searching for clues regarding the history of the FEV virus. Throughout years, he and his army led a guerilla war against the Brotherhood, eventually forcing them to start a crusade in order to annihilate the remaining super mutant threat.

Attis learns about a secret Vault-Tec installation with new and possibly improved versions of the FEV virus. While it is unknown where he discovered such an information, a possibility is that he found another Vault-Tec facility with the information available to extract. He led his group to the ruined city of Los, where the Secret Vault was located. His ultimate goal was the continuation of The Master's plan - to rebuild the mutant army and develop a cure to the mutant sterility. The Secret Vault gives him hope, as the Vault scientists' researches was dedicated to improving the original FEV in order to maintain reproductive functions. However, when Attis found this FEV and tested it on himself, it was revealed that this FEV is unable to make mutants fertile. However, an unforseen consequences followed, revealing another potential of the modified FEV.

Confrontation with the Initiate
During their first encounter with the Brotherhood of Steel Initiate, who comes to the Vault to destroy the super mutants, Attis uses his blade to fight. Though he loses, the nightkin appear and quickly capture the Initiate. Attis cut off his arm and commanded the nightkin to throw him into the pit. However, thanks to Mary, the Initiate managed to survive and later even recieved a new arm through regeneration experiment.

Much later, in the end of game, Initiate encounters Attis again, this time in the Vault laboratory core. Now, Attis is equipped with a Gatling Laser, a big laser gun, and a stealth device. He tested the new FEV on himself, but it only led to a strange side effect and no fertility. Attis now has a strange organic biomass growing out of his back and head. The fight begins, but due to exposure to Vault-Tec's modified FEV, he increases in size significantly mid-way through the battle, reaching the size that of a Super Mutant Behemoth. Though initially he believed his discovery leads to nothing, now he begins to realize the new potential of the virus.

The new Master
The modified FEV proves highly unstable, causing Attis to regenerate uncontrollably due to amount of damage he recieves. He explodes into a living gooey biomass that quickly multiplies and covers the entire lower floors of the Vault. This new creature begin to grow with each second, and it's various organs, like eyes, mouth, and tentacles begin to appear out of the floor. This organic nightmare retains some consiosness of Attis, now starting to dream of a new world, similar to the Master's Unity. In this gigantic form, Attis tried to make remaining surivived super mutants found in the Vault to "join" him, but they were scared of him and began to fight him back.

The Initiate ran from him, fighting his tentacles, his giant "heads" with one eye, and several fleshy walls with big mouth in the center of them. Eventually, he found Patty, the Vault security officer, who now was enveloped in a cocoon-like state by Attis and is slowly eaten by him. She reveals that there is a way to start a self-destruct sequence, and asked the Initiate to kill her. In the next room, the needed terminal was found, and the Initiate escaped using monorail, while the Vault's nuclear warhead, specially designed for this purpose, detonated, killing Attis forever.

Appearances in games
Attis appears only in Fallout: Brotherhood of Steel. While the game itself is not canon, such faction did indeed exist, as can be seen in the remnants of the Master's army encounters in Fallout 2. Attis was voiced by Tony Jay, who previously voiced the Lieutenant in Fallout.