Elijah, also known as Father Elijah or the Father, was a rogue Brotherhood of Steel Elder indirectly responsible for the Fall.
Elijah was a savant amongst the Brotherhood of Steel's members, rising to the rank of Elder as a Scribe in an unprecedented move. Assigned as Elder of the Mojave Chapter, he came into conflict with more traditionalist members of the organization due to his view that technology should be researched and improved rather than just preserved. He took on a young Veronica Santelego as his apprentice, instilling in her a more progressive view of the Brotherhood's potential, but as the years passed, he became more unstable and obsessed with his vision, becoming distant even from his pupil. As he grew more controlling, he alienated Veronica by separating her from her lover, Christine Royce, resulting in Royce's reassignment to the Circle of Steel. This culminated when the Brotherhood discovered Helios One in the Mojave- believing it to be a superweapon, Elijah expended personnel and resources in failed attempts to activate it, ending in Operation Sunburst in 2274, a massive NCR attack on the small Brotherhood garrison. Determined to protect the site, Elijah sacrificed the lives of over 100 BOS members in a futile attempt to defend the site, choosing to abandon his brethren as the tide turned in the NCR's favor. The Mojave Chapter survived only due to the decision by Paladin Nolan MacNamara to retreat back to Hidden Valley.
On the run from the NCR and eventually his former comrades, Elijah set about a trail of crimes across the American Southwest, hunting lost Pre-War knowledge as he developed a twisted plan to restore the Brotherhood's power and destroy the NCR. Eventually he came to the Big MT Research Facility, where he discovered numerous unethical technologies as well as a supply of Slave Collars, and learned of the Sierra Madre Casino. He remained in the crater for an extended period, until the machinations of the think Tank and Christine Royce's arrival forced him to flee big MT by train.
Elijah traveled to the Sierra Madre, but found that it was guarded by the Cloud and a complex security system that required the effort of several people to crack. He managed to gain command over a mentally-damaged Nightkin, Dog, using the mutant to capture victims for a "team" to breach the Sierra Madre, fitting them with slave collars to ensure obedience. They however, inevitably turned on each other out of greed or insanity, forcing Elijah to link the collars to self-destruct if a team member died. One of these missions partially succeeded, but the team was killed by the Sierra Madre's hazards and Elijah became trapped within the Casino itself.
Roughly around this time, the Courier, investigating the Sierra Madre's radio signal on their Pip-Boy, was lured into one of Elijah's traps and taken to the Casino by Dog. Forced to work with Dog, ghoulified Prewar "King of Swing" Dean Domino, and Christine Royce, the Courier successfully breached the Madre and through unknown circumstances killed the other three team members.
What occurred next is unknown, but Elijah survived his encounter with the Courier- whether the Courier aided him in his plans or he managed to successfully escape the Sierra Madre Vault after being locked in it is unclear.
After the Second Battle of Hoover Dam, Elijah explored the Madre and took control of many of its systems, becoming known as "the Father," a psuedo-god figure to the Ghost People. His plans were disrupted however, when an NCR expedition supported by the Dead Bears Tribe came upon the Madre. Effective tech scavengers, the Dead Bears scavenged Darklight Hazmat Suits from the villa ruins, under the supervision of a small contigent of NCR troops. Realizing the tribe were former Legion members, Elijah contacted them and managed to convince them to turn on their NCR overseers, giving them samples of the Cloud to pass on to the NCR, confident that the Republic would ensure its own destruction with the toxic substance. This proved right, as an unknown incident led to the Cloud covering much of NCR's territory and destroying the cities of New Adytum and New Vegas. As the Fall occurred, Elijah led the dead Bears in a violent uprising against their NCR superiors, during which surviving NCR forces sealed the Sierra Madre gate with an old tuck. Elijah branded his new followers the Cloud Walkers and proclaimed the NCR's destruction, but trapped in the Sierra Madre, he and his new tribal allies eventually succumbed to the Cloud and starvation.
Eliajh's skeleton, complete with rotting BOS Elder robes, can be encountered at the Villa Fountain in the Sierra Madre alongside the bodies of his Cloud Walker allies, next to several holodisks recording his time in the Sierra Madre. His body has a high-condition Gauss Rfile and several microfusion cells on it.