Attis

Attis was a super mutant, one of the Master's soldiers, and after the death of the Master he became a leader of the faction which tried to continue the Master's work, rebuild the army, and dominate Southern California once again. 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. Learning about a secret Vault-Tec installation with new and possibly improved versions of the FEV virus, Attis led his group to the area where the rumors started, the ruined city of Los. His ultimate goal was the continuation of The Master's plan - to rebuild the mutant vats and eventually transform the entire human race into mutants. His forces were eventually defeated by the Brotherhood of Steel.

Attis is equipped with a Gatling Laser. Due to exposure to Vault-Tec's modified FEV, he increases in size significantly mid-way through the game's final battle. However, the FEV proves highly unstable, causing Attis to mutate uncontrollably and eventually explode into a living gooey biomass that covers the entire lower floor of the final Vault (his various giant, blob-like, mutated organs (i.e. eyes, mouth, etc.) serve as the game's final enemies).

Attis was voiced by Tony Jay.

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, so Attis at the faction leader's name can just as well be used, although Vault-Tec's FEV installations are definitely not canon.