Xylourgos is an icy planet located in the farthest reaches of the galaxy, overshadowed by an eternal eclipse. On the surface of the planet lies Gythian, a deceased Vault monster whose heart still beats on even after death.
In the distant past, the Eridians used the planet of Xylourgos to imprison a unique Vault monster named Gythian, who possessed the power to influence the hearts of others and grant eternal life to those under his control, though at the cost of their humanity. Gythian would be locked away in a Vault on the planet's treacherous mountain pass of Negul Neshai, though Gythian would somehow escape his prison. By unknown circumstances, Gythian would be slain; his body now laid dead on the surface of the planet. Despite this, Gythian still lived on as his heart continued to beat, even as his physical body laid dead, and began influencing the world around him with his mysterious powers.
Around the time when the Atlas corporation made its findings of Eridian technology within a Vault on Promethea, the Dahl corporation began its own universal search for other Vaults to further their own success and corporate dominance like Atlas. A Dahl research team under the command of Eleanor and Vincent Olmstead would travel to Xylourgos to research Gythian, whose legend vastly interested the Olmsteads. The research team would conduct various research on Gyhtian's physiology, as well as the mutagenic and metaphysical properties of his power, hoping to discover new xenobiological revelations that would change the universe. Upon investigating the Eridian ruins at the former site of Gythian's Vault on Negul Neshai, the Olmsteads would discover the same Eridian technology found within the Vault found by Atlas, and Vincent would find a connection to Gythian's power and the Eridian technology, wishing to experiment with this discovery further.
Vincent's experiments however would lead him to slowly fall under Gythian's influence, driving him mad and influencing him to sacrifice his humanity in exchange for eternal life. Now completely obsessed with power, Vincent would travel alone to Gythian's heart chamber, only to be entombed within the heart itself; Vincent however would remain alive, but was now unable to leave his bonds physically. Eleanor, who was too late to stop Vincent, began devising a plan to free her beloved; using Vincent's research on the connection between the Eridian technology and Gythian's heart, Eleanor forged two rings from the Eridian technology and began work on refining a sample of Gythian's heart she recovered by utilizing their research ship's reactor. Upon finishing her work, she infused the rings with the sample of Gythian's heart, and upon putting one of the rings on her own finger, she too began to become influenced by Gythian.
As she was considered half of the bond, she needed one more to wear the other ring intended for Vincent, which would draw out his mind from Gythian's heart at her behest and into the wearer of the other ring. Upon placing Vincent's ring on the finger of her fellow crew member Harrington, he was soon possessed by Vincent, who used his body as a new vessel while his original body remained within Gythian's heart. However, the bodies Vincent would possess would only last for so long before dying, forcing the Olmsteads to form a scheme to lure people to Xylourgos to supply Vincent with more vessels. Revealing Xylourgos to the rest of the universe, the Olmsteads drew countless researchers, tourists and various others to Xylourgos and their newly-created town, where they are subjected to Gythian's influence by the Olmsteads. The Olmsteads would then form the Bonded, a cult-based following who worshiped Gythian, and would act as the many vessels that would be supplied to Vincent. As for the remaining inhabitants of the town, they would become cursed by Gythian's power as a result of their prolonged stay around Gythian’s body, gaining abnormal traits and uncontrollable powers.
Roughly 60 years later since the start of the Olmsteads' expedition, Xylourgos is selected by Gaige to be the wedding destination for the marriage of Wainwright Jakobs and Sir Hammerlock, oblivious of the threat posed by the Bonded. Prior to the beginning of the festivities, the Vault Hunters and Wainwright venture through Cursehaven and stumble upon a Bonded Renewal ritual, which was in the process of granting Vincent a new vessel. In the process, the Vault Hunters defeat Vincent, though his ring possessing his previous host falls off and forces itself onto Wainwright. Eleanor deems Wainwright to be the perfect vessel for Vincent and attempts to reinforce his control over Wainwright's body, though she is stopped by the timely arrival of Gaige and Deathtrap, who assist the Vault Hunters in escaping with Wainwright. Undeterred, Eleanor remains confident that Wainwright would eventually succumb to Vincent's possession.
Wainwright is brought to The Lodge, where the structure's mysterious properties allows him to resist Vincent's possession at a slower rate, though it would not last. To save Wainwright, the Vault Hunters are encouraged by the enigmatic yet humble Mancubus Bloodtooth to seek out Burton Briggs, a paranormal investigator in Cursehaven. At Burton's suggestion, the Vault Hunters venture to the Dustbound Archives to gain knowledge of the Olmsteads' past and the origins of their rings. At the archives, the Vault Hunters locate a Delta-3 tape and bring it back to The Lodge, where they play it back to learn that the power of the Olmsteads' rings is derived from Gythian, and the only way to neutralize their power was to destroy Gythian's heart. With this knowledge, the Vault Hunters sought to recover the original heart sample Eleanor obtained from Gythian, which they could use against Eleanor and her forces. On their way to the sample's location at the abandoned Dahl research site on Negul Neshai, they encounter Eista and his tribe, who inform the Vault Hunters that the path to Negul Neshai was blocked by a powerful ice and that the only way to pass through was to recover the kife of the Wendigo. Eista directs the Vault Hunters to The Cankerwood, where they are assisted by Hammerlock in hunting down and killing the Wendigo, recovering its kife in the process.
With the path to Negul Neshai clear, the Vault Hunters venture through the treacherous mountain pass until they reach Eleanor's ship. With the assistance of Deathtrap, the Vault Hunters recover the heart sample, and are instructed by Gaige to force it into Deathtrap, who would be given a newfound strength capable of piercing through the barrier encasing Gythian's heart chamber. However, their efforts are seemingly cut short when Deathtrap is overloaded with too much power and presumed destroyed, much to Gaige's despair. However, Deathtrap is eventually reborn as Deathtrap 2.0, and reveals himself during the Vault Hunter's fight with the Empowered Grawn. After defeating the Bonded elite, the group learns from Hammerlock that Wainwright had finally succumbed to Vincent, and proceeds to give chase to his possessed love to the wedding venue, though they would both end up being captured by the Bonded. The Vault Hunters, Gaige and Deathtrap all venture to Heart's Desire and neutralize the barrier protecting the site, allowing the Vault Hunters to gain access to Gythian's heart chamber. There, the Vault Hunters fight Eleanor and Gythian's heart, and in the process destroy the heart, freeing Wainwright from Vincent's influence and simultaneously Vincent from his captivity within Gythian's heart. While Vincent and Eleanor share one last moment before dying together, Wainwright and Hammerlock follow through with their marriage and are wed by the Vault Hunters within the heart chamber, using the powerless rings that once belonged to the Olmsteads as their wedding rings.