Galenth Dysley is introduced as the primarch of Cocoon's Sanctum government in Final Fantasy XIII. Under his command, the Sanctum military branch PSICOM begins a purge of the city of Bodhum after a Pulse fal'Cie is found, and sits back as PSICOM hunts down Lightning and the others that had been made into l'Cie by the fal'Cie's power. He is on board the bridge of the Palamecia, a Sanctum airship, as PSICOM officer Jihl Nabaat enacts a plan to gather all of the l'Cie in one place. However, as the l'Cie wreak havoc throughout the ship and join forces, Dysley remains unconcerned, even as the normally cold, calculating Nabaat begins to crack under pressure. When the l'Cie arrive on the bridge, Dysley surprises everyone by killing Nabaat and the remaining bridge crew before Nabaat can deal with the l'Cie personally.
It is at this point that Dysley drops his human facade. He is in actuality Barthandelus, a Cocoon fal'Cie. Disguised as a human primarch, he had manipulated events from behind the scenes. The goal of Barthandelus is the death of Orphan, the being that grants energy to Eden; the fal'Cie that keeps Cocoon stable. By extension, the death of Orphan would also result in the destruction of Cocoon. Though he would die as well, Barthandelus believes that such an act would purify Cocoon and summon the return of the Maker to create the world anew. However, because Cocoon fal'Cie and l'Cie are incapable of killing Orphan, he needs Pulse l'Cie, in effect, Lightning and the others, to do his work for him.
Barthandelus pursues this goal by goading the l'Cie into completing their Focus. He keeps the Sanctum military on their tail, and uses illusions such as disguising himself as Lightning's sister Serah in order to play on their emotions. The l'Cie, with little choice but to follow the completion of their Focus or be cursed to become Cie'th, eventually make their way to Orphan's Cradle, where Barthandelus is there to welcome them. However, when the party refuses to do as he bids and kill Orphan, he attacks them, only to be defeated.
Upon his defeat, Barthandelus falls into Orphan's pool. His body merges with Orphan, who awakens to confront the l'Cie. As it turns out, that Barthandelus and Orphan shared the same goal all along, but this merged incarnation of Barthandelus and Orphan is also defeated, leaving only Orphan itself to be dealt with.