|The following is based on Fallout: Brotherhood of Steel and has not been confirmed by canon sources.|
Attis was one of the commanders of the faction of super mutants who fled to the East after the death of the Master and the destruction of Mariposa Military Base. Starting to talk about themselves, Rhombus, now High Elder of the Brotherhood of Steel, thinking they were a threat, demanded during the elder council that the Brotherhood look into it. But the elder council answered in the negative, because the former expedition grew by similar ambitions (track down and assess the extent of the remaining super mutant threat) against the Gammorin's Army, and the incident ensuing divided the Brotherhood, creating a splinter faction of the Brotherhood of Steel.
But Rhombus didn't listen to the elder council and started a crusade against the Attis Army. With some Scribes and Paladins, he mounted a new expedition which was later named Texas Expedition and began tracking the Attis Army. They followed their trail to Texas and there, he discovered the Vault Prototype, an abandoned vault prototype dedicated to the Secret Vault project and installed the main base of the Brotherhood of Steel in Texas.
The Attis Army searched for clues regarding the history of FEV before trying to make the Master's plan a reality, and learned about a secret Vault-Tec installation called the Secret Vault, the Vault-Tec private Vault after the Great War, with new and possibly improved versions of the FEV virus. During the travel, the army came across the Carbon Mill and encountered Jane, the raider matron of the largest and powerful raider gang of Texas, who offered to sell slaves regularly.
After that, the army traveled to the ghoul city of Los in Texas, where the Secret Vault was located. Their first goal was to rebuild the mutant army and develop a cure to mutant sterility. The Secret Vault gave Attis hope, as the Vault scientists' research was dedicated to improving the original FEV in order to maintain reproductive functions.
Arriving in Los, Attis attempted to negotiate with the Church of the Lost, the cult that controls the city, but in vain. Just some time later, a contingent of Paladins of the Brotherhood of Steel lead by Rhombus arrived and a battle broke out between the Attis Army, the Church of the Lost and the Brotherhood Paladins. The Brotherhood of Steel group was annihilated and Rhombus was captured. Attis used this confusion to his advantage and managed to get into the Secret Vault. However his army remained blocked at this level because they couldn't gain access to the lower level, which required a key card.
Triggering a siege on the city to get the key card and going to war with the Church of the Lost, Attis continued to prepare his plan. Later in 2209, the Initiate came into Los, destroyed a weakened Church of the Lost, killed Blake (the high priest of the cult) and came to the Secret Vault to destroy both the Attis army and the Secret Vault with the key card stolen and hidden by Rhombus. The Initiate killed many super mutants of Attis' army and entered the Hub chamber of the residence area.
Refusing to give him the key card, Attis engaged the Initiate in combat, using his retractable arm blade to fight. When he lost, two nightkin appeared and quickly stunned and captured the Initiate. Taking him to an area with a huge pit down to the ruins, Attis took the key card, cut off the Initiate's arm and commanded the two nightkin to throw him into the pit. A few moments later, the Initiate encountered Mary in the ruins, who helped and lead them to the garden, where all survivors of the vault now lived. When they arrived, the survivors gave the Initiate a new arm with a restoring organic table.
Attis had penetrated deeper into the Vault, until the last section of the laboratory, and in the laboratory core he had found the Vault-Tec modified FEV. Just before the Initiate arrived and combat started, he tested it on himself, and found that this FEV is unable to make mutants fertile. However, an unforeseen consequences followed, revealing another potential of the Vault-Tec modified FEV; it led to a strange side effects and his skin turned darker. During the battle, Attis exploded and decomposed into a strange organic biomass, a rapidly-multiplying multicellular organism that covered the entire laboratory section of the Secret Vault.
In this gigantic form, Attis tried to make the rest of the Attis Army found in the Secret Vault "join" him, but they were scared of him and began to fight back. After the Initiate found the Vault's nuclear warhead, he triggered it and escaped with a monorail, destroying the Attis Army, Los and the Secret Vault.
The Attis Army appears only in Fallout: Brotherhood of Steel.