Hayden works his way to the entrance of the Vault to rendezvous with the A.D., who says he has made a deal with Mezner and gives Hayden a booster "for the road". Outraged from being used and betrayed, Hayden stabs him in the neck with the booster, telling him that he now feels "better than ever", and kills all of the A.D.'s men before heading for the Vault. Finding Yargo, Hayden gives him the key, telling him to seal the Vault and dispose of the key. Inside the Vault, a stunned Hayden discovers the first known source of the Technocyte virus: an American submarine that surfaced off the coast of Lasria (seen in the prologue of the game). Hayden discovers Mezner with the Technocyte transmitter, a Hydra-like monstrosity. After fighting and defeating Mezner, the monster and several infected, Yargo arrives to tell Hayden that the transmission is still going out. Hayden tries to fry the circuitry with his Glaive; but Mezner, not yet dead, stuns his right arm, telling him: "You are one of us now." With his right hand immobilized, Hayden catches the now-electrified Glaive with his left hand, and impales Mezner's skull with it. With the transmission finally halted, the game ends with Hayden leaving the Vault, catching the Glaive as he steps outside. Yargo, who apparently survives, narrates: "That was how it started, the irony of this disease. That in all the others, it made evil; but for "him" , it had saved his soul."