|The following is based on Fallout: Brotherhood of Steel and has not been confirmed by canon sources.|
Attis is a highly intelligent super mutant and was an important commander in the Master's Army. After the destruction of the Master and the vats, he created the "Attis army" from remnants of the Master's army. He then began searching for clues regarding the history of FEV before trying to make the Master's plan a reality.
Attis is one of the few super mutants with great strength, both intellectual and physical like the Lieutenant or Marcus. He is tall, strong and quite athletic for a super mutant. He is also very fast and knows how to use all kinds of weapons. After the death of the Master and the destruction of the vats, he created and became the general of the faction which tried to continue the Master's work; rebuild the army and transform the entire human race into super mutants, to have a single race able to survive - thrive in the wasteland, eliminating any differences and removing all discord.
After Attis created his own army, he took his group into hiding, searching for clues regarding the history of the FEV virus. Throughout the years, he and his army led a guerrilla war against the Brotherhood of Steel, (still looking for the rest of the mutant army because of the failure of the Airship's expedition that created the Midwestern Brotherhood of Steel), eventually forcing Rhombus to start a crusade named "Texas Expedition" in order to annihilate the remaining super mutant threat.
Attis 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. He led his group to the ghoul city of Los in Texas, where the Secret Vault was located. His first goal was to rebuild the mutant army and develop a cure for mutant sterility. The Secret Vault gave him hope, as the vault scientists' research was dedicated to improving the original FEV in order to maintain reproductive functions.
Arriving in Los, he tried to negotiate with the Church of the Lost, the cult that controlled the city, but in vain. Some time later a contingent of paladins of the Brotherhood of Steel, lead by Rhombus, arrived and a battle broke out between 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.
Besieging the city to get the key card and going to war with the Church of the Lost. He continued to develop his plan and allied himself with Jane, the raider matron from the largest and most powerful raider gang in Texas located in Carbon. She sold him human slaves from raids in Carbon, and he goes into Carbon himself to retrieve them personally. They now became slaves of Attis but their function in his army remains unknown.
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, killed Blake, the high priest of the cult and came to the Secret Vault to destroy both Attis' army and the Secret Vault with the key card stolen and hidden by Rhombus, preventing Attis from exhausting his army in a long drawn-out siege. Refusing to give him the key card, Attis engaged in combat, using his retractable arm blade to fight. Though he loses, two nightkin appear and quickly stun and capture the Initiate. Taking him to an area with a huge pit down in the Secret Vault ruins. Attis took the key card, 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 received a new arm through a regeneration experiment.
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 combat started, he tested it on himself. It was revealed that this FEV is unable to make mutants fertile. However, an unforeseen consequence followed, revealing another potential of the Vault-Tec modified FEV, it led to a strange side effect and his skin turned darker. During the battle, he mutated again, growing and reaching the size of a super mutant behemoth and all his injuries were healed. After being seriously injured again by the Initiate, the modified FEV proved highly unstable, causing Attis to regenerate uncontrollably due to the amount of damage he received. He exploded and coalesced into a strange biomass, a rapidly-multiplying multicellular organism that covered the entire laboratory section of the Secret Vault. Though initially he believed his discovery lead to nothing, soon he began to realize the new potential of the virus.
This new creature began to grow with each second, and its various organs, eyes, mouths and tentacles began to grow out of the floor. This organic nightmare retained some of Attis' conscious, starting to dream of a new world, similar to the Master's Unity. In this gigantic form, Attis tried to make the remaining super mutants in the Secret Vault "join" him, but they were scared of him and fought back. The Initiate ran from him, fighting his tentacles, his giant "heads" with one eye and several fleshy walls with big mouths in the center of them. Eventually, he found Patty, the vault security officer, now enveloped in a cocoon-like state by Attis, being slowly digested. She revealed that there was 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 the vault's monorail, while the vault's nuclear warhead, specially designed for this purpose, detonated, killing Attis.
- Attis seems to collect the skulls of some of his victims. Two skulls are seen hanging on two sticks he carries on his back and two others are hanging in his hair. He also expresses a desire to collect the player character's skull should they anger him.
- While his mutations due to the Secret Vault modified FEV transformed him into a multicellular mutant similar to the Master, they are still different creatures.
- As can be seen in the remnants of the Master's army encounter in Fallout 2, such factions did indeed exist.
- Attis resembles the Lieutenant from the original Fallout. Both are highly intelligent super mutants with cybernetic implants over their right eye, and even have the same voice actor, Tony Jay.
- "Yes, we are many. A mutant army. The destruction of our Master has only strengthened our resolve. We will control this vault - and its scientific secrets - then use it as a base of operations for our war on humanity"
- "Ah, at last I meet the mouse who stalks lions. I am Attis, a general in the mutant army... and you have something I need, little mouse"
- "Your humans are easily manipulated. After the unfortunate business with the psycho ghouls, it was easier to let you bring me the key rather than enter a long, drawn-out siege on the city. I have other plans for my army"
- "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!"
- "I will restore the dead world, blanketing it with life, removing all discord. It will be a perfect world"
- "And it was given unto the beast to make war with the saints, to overcome them... and power was given him over all kindreds and tongues and nations..."
Attis appears only in Fallout: Brotherhood of Steel.