Father Elijah is the main antagonist of the Fallout: New Vegas expansion Dead Money. He was the leader of the Brotherhood of Steel in the Mojave Wasteland, but prior to the start of the game he got most of the Brotherhood killed by the NCR at the HELIOS One power plant while trying to find the ARCHIMEDES laser weapon, and was then fired and disappeared and hasn't been heard from since. Veronica Santangelo, one of the Courier's companions, looked up to him as a father figure and believes he is still alive and doing something important.
Upon installing Dead Money, the player picks up a mysterious radio signal advertising the grand opening of a casino called the "Sierra Madre" coming from an abandoned Brotherhood of Steel bunker near the eastern edge of the map. Once inside, the Courier is knocked out by sleeping gas that is released when they walk into the room with the radio. A cutscene then plays explaining that the Sierra Madre was designed to be the ultimate casino, but never opened due to nuclear war breaking out and has been a mythical place to denizens of the wasteland ever since.
When the player wakes up, they are in the middle of the abandoned city surrounding the Sierra Madre and are greeted by a hologram of Elijah who tells them that they have to help him break into the Sierra Madre's vault or else he will detonate the explosive collar around their head. He instructs the player to find three other people with collars like theirs: Dog, a Nightkin with a particularly severe case of split personality disorder, Dean Domino, a ghoul who used to be a lounge singer and was going to perform at the Sierra Madre when it first opened, and Christine, a Brotherhood scribe who was rendered mute and bald after being hurt by a malfunctioning Auto-Doc, and is also implied to be Veronica's estranged lesbian girlfriend mentioned earlier in the game.
Once these three are brought together, they are able to trigger the scripted "Grand Opening" event and open the gates into the Sierra Madre resort, and once inside they are mysteriously separated. Elijah then tells the player that the others are no longer necessary for the plan and actually encourages you to kill them, but all that is required is that all three are located and accounted for before meeting Elijah in the Sierra Madre vault.
When the Courier meets up with Christine, she has downloaded the voice of the casino's previous caretaker, Vera Keyes, into herself and is finally able to tell the player the whole truth about Father Elijah, revealing that after leaving he started looking everywhere he could for new technology for the Brotherhood, and started resorting to crime and violence to achieve that end. Christine was sent by the Brotherhood to stop him and confronted him in a factory called the "Big Empty", but Elijah managed to subdue her and implant cybernetic modifications into her brain, enslaving her. In addition, Elijah is actually trapped in the Sierra Madre's vault room just outside the treasure chamber, and needs just one more person to reach it to free him and give him access to the vault. Christine wants to help the Courier kill Elijah.
Inside the vault chamber, Elijah talks to the player again through a computer screen and reveals his true plans for the Sierra Madre, which is to use its technology to take over the Mojave Desert; such as the invincible security holograms that could single-handedly take on an army of men, the vending machines that can generate supplies from thin air, and the poisonous gas cloud surrounding the resort. No longer needing the Courier's help, Elijah then activates the Vault's security turrets to kill them, but they are destroyed and Elijah is exposed and killed. Alternatively, you can convince him to come down into the vault without him activating the turrets, but he will activate a trap in the computer files set by the founder of the casino and be trapped inside the vault forever.
Fallout: New Vegas