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Exodus Siege is an Adventure located on Nessus. It is unlocked in tandem with all other adventures on Nessus after completing the mission Six. This adventure is in a prelude to the strike Exodus Crash.
During the course of Exodus Siege the Fallen attempt to destroy the Exodus Black and Failsafe in order to melt it down into scrap to scavenge, however the player manages to stop them and drive off the assault. The player then follows the Fallen transmissions back to their source in the Chamber of Sky where Elyksul, Assault Leader is discovered to be orchestrating the assault from behind the safety of several invincible shields derived from Vex technology. Evidently the player failed to destroy all such shield generators during the adventure Unbreakable, and the Fallen, led by Elyskul have managed to steal the technology and use it to their own ends. The player uses Arc Charges to overload the shield generators, get at Elyksul and kill him, ending the assault and saving Failsafe and the Exodus Black.