Perhaps you think this is a simple robbery. A cheap casino heist. No... this is a heist of the centuries. We're not plundering the Sierra Madre - we're plundering history, taking from the Old World itself.”— Elijah
The Heist of the Sierra Madre was the infiltration and raiding of the Sierra Madre Casino in the year 2281, with the objective of acquiring the advanced Pre-War technology inside the Sierra Madre, releasing the Cloud upon the Mojave Wasteland, and for Elijah to establish a new order in the Mojave Wasteland using the hologram and molecular assembler technology from the Sierra Madre. The heist failed, and Elijah was either killed or trapped inside the casino vault, and the Courier escaped, as well as possibly some of the other three prisoners.
After the Brotherhood of Steel was defeated by the NCR at HELIOS One, Father Elijah disappeared into the wastes to search for technologies that would save the Brotherhood - to find what he believed was "one of the greatest treasures of the Old World." For Elijah, the Brotherhood was doomed; they needed to begin again and he would do whatever was needed, no matter how morally questionable, to ensure that came to pass. He traveled throughout the Wasteland, going as far as the Divide and spending time among the Ciphers.
Eventually, Elijah stumbled across the Big MT in his travels, where he found an incredible amount of rare pre-War technology. Captured temporarily by the Think tank upon his arrival, he managed to escape in seconds, much to the shock of the brain bots. In the great expanse of Big Mountain, Elijah ran into another human, a courier, who spoke to him of the legend of the Sierra Madre. Making his way to Little Yangtze, he experimented on the survivors of the prison camp with explosive collars while, curiosity getting the better of him, seeking out the radio frequency until finally tracking down a faint broadcast. Avoiding the local robots and suffering from migraines through the overuse of Mentats, Elijah fine-tuned both the collars and the radio signal. Now, with the location of the mythical casino and his collars ready, his grand plan to defeat the New California Republic was ever so close to fruition.
However, the trail of crimes he had left across the wastes in the years preceding his arrival had not gone unnoticed, and a hardline faction of the Brotherhood soon dispatched assassins to stop him. Christine Royce was one of them. As the Think Tank sent robots to contain him and, catching a glimmer from a sniper scope, Elijah instinctively sensed the worst. He held out in the tower for a while, sniping the Think Tank's robots with his Tesla Cannon as they funneled through. Then, strategically detonating his subjects' collars to make his getaway as she attempted her assassination, Christine was defeated, becoming a subject for medical experimentation as she was dragged away to the Y-17 medical facility. Cybernetic modifications, one of the many Old World horrors left in the Big Empty, were implanted in her brain that damaged her perceptions of the world around her. Hacking into the Think Tank mainframe as the Big MT's robots closed in, Elijah fried Doctor 8's voice module and rerouted Doctor 0's processors to take control of the train network. Via remote control, he directed a train into one of the tunnels, creating a path for his escape through the wreckage. Leaving his retrofitted LAER rifles and jury-rigged Tesla cannon behind, he then set off towards the Sierra Madre with a Gauss rifle, several explosive collars, a Holorifle and, crucially, the knowledge that could make the Mojave his, and his alone.
To prepare for his expedition, he renovated an abandoned Brotherhood of Steel bunker as a base of operations in the Mojave, setting it up ready for when he managed to get his hands on the technologies it offered. In making round-trips around the Sierra Madre to his bunker, he performed analysis of the Cloud and the Sierra Madre's environment. Once ready to go to the Sierra Madre, he left a final message for his protégée, Veronica Santangelo, in the event he did not return.
Elijah captured many wastelanders to use as slaves to complete the heist, all of whom eventually died or killed each other, excluding Christine, Dog and God and Dean Domino. Elijah eventually captured the Courier. The Courier collected the three other prisoners who had survived, and triggered the Gala Event in the Villa. After this, the Courier and their three companions moved to the casino, where the casino's security features rendered all of them unconscious. Christine Royce was taken to the Executive suites. Dean Domino was taken to The Tampico, and Dog and God was taken to the Cantina Madrid. The Courier woke up in the lobby, and after reactivating the casino itself, dealt with their companions one by one. In the Tampico, the Courier either rescued Dean or was forced to kill him. In the executive suites, the Courier could either get help from Christine or be forced to eliminate her. In the Cantina Madrid, the Courier could have outright killed Dog and God. They could have had Dog take control, or have God take control. Or, the Courier could have merged them into one personality. During this time the ghost people made into the casino, and the Courier was forced to fight them. After all this was done, they moved down to the vault. After getting by numerous holograms, speakers, and portions of the cloud, the courier arrived at the vault. There are numerous outcomes as to what can happen here, but two out of the three are considered non-canon. Elijah will eventually come down to the vault and attempt to deal with the Courier. If Christine is on good terms with the Courier at this point, she will deactivate the turrets that are hostile to them. If Dean is on good terms with the Courier, he will deactivate the vault's speakers. No matter how the battle played out, Elijah was defeated and the Courier escaped. The Courier could just have killed him outright or trapped him in the vault. After this, the Courier leaves the Sierra Madre behind, optionally escaping with many of the Madre's treasures. But what Elijah was after, holograms and a way to weaponize the cloud, were not obtained, rendering the heist an overall failure.
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