|The following is based on Fallout: Brotherhood of Steel and has not been confirmed by canon sources.|
Fertility. That's what we came for. The research in this vault was centered around curing sterility due to mutation. Imagine, then, a treatment that allowed mutants to reproduce, to breed. That would truly be a weapon!”— Attis
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 and demanded during the elder council that the Brotherhood looking after them. But the elder council answered in the negative, because of the former expedition grow by similar ambitions (track down and assess to the extent of the remaining super mutant threat) against the Gammorin's Army, and the incident ensuing that divided the Brotherhood, created a splinter faction of the Brotherhood of Steel.
But Rhombus, didn't listen to the elder council and starting 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 searching for clues regarding the history of FEV before trying to make the Master's plan a reality, and learns 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 come across the Carbon Mill and encounter Jane, the raider matron of the most large and powerful raider gang of Texas, who offered to sell slaves regularly.
After that, the army travel 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 gives him hope, as the Vault scientists' researches was dedicated to improving the original FEV in order to maintain reproductive functions.
Arriving in Los, Attis tried 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. But to access the laboratory section, he had to get a key card, only possessed by the Church of the Lost.
Triggering a siege on the city to get the key card and going to war with the Church of the Lost, the Attis Army continued to prepare his plan. Continuing his plans for the war against the Church of the Lost and to get the key card, the Initiate changed the deal, destroyed a weakened Church of the Lost, kill Blake the high priest of the cult and comes to the Secret Vault to destroy both the Attis Army and the Secret Vault with the key card stolen and hidden by Rhombus, preventing Attis to exhausting his army in a long drawn-out siege.
Refusing to give the key card, Attis engaged the combat, using his retractable arm blade to fight. Though he loses, two nightkin appear and quickly stuns and capture the Initiate. Taking him to an area with a huge pit down to the Secret Vault ruins, Attis took the key card, cuts off his arm and commands the nightkin to throw him into the pit.
Attis penetrates deeper into the Vault, until the last section of the laboratory, the laboratory core and found the Vault-Tec modified FEV. Just before the Initiate arrived and the combat start, he tested it on himself, it was revealed 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 effect and his skin turned more darker. During the battle, Attis explodes and decomposes into a strange organic biomass, a rapidly-multiplying multicellular organism that covers 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 find the Vault's nuclear warhead, he triggered the nuclear warhead and escaped with a monorail, destroying the Attis Army, Los and the Secret Vault.