Abbadon is an immense monster that was was created by Orfeo and defeated and caged in the Southern Sea.
Abbadon first appears in the prologue where the Iele are trying to contain it in waterfire after he broke through their ochi spell. It whispers 'deceiver' in many languages before the Iele manage to push it back through the waterfire.
Sera and her Black Fins go to the Southern Sea to defeat Abaddon. They enter the prison after killing Orfeo, who opened the prison, the prepare to fight him. Abbadon lashes out and strikes at Ling, wounding her right side. While Becca song casts to keep him occupied, Ling tells Astrid that Abbadon is made of souls. Astrid sings a funeral dirge to release the souls from Abbadon, destroying him. The souls are collected in pearls from Astrid's necklace. Horok collects the pearls, including Orfeo's one, after the battle.