Eris is the goddess of chaos, and as such, tries her best to make the world chaotic. She controls an army of huge, savage monsters, including a Kraken, a Roc, and a giant scorpion. Since she is a goddess, Eris has almost omnipotent-level powers (though by no means invincible). Eris is first seen watching Prince Proteus sailing towards Syracuse with the Book of Peace on board his ship, with the thief Sinbad in hot pursuit. She sends her loyal sea monster Cetus to attack the ship so she can acquire the Book, but Proteus and Sinbad work together to defeat the beast. However, Cetus drags Sinbad into the depths of the sea where he meets Eris, who breathes a huge underwater bubble to allow him to breathe. Eros promises to give Sinbad anything he wants in exchange for stealing the Book of Peace for her, which he agrees to.
Sinbad and his crew travel to Syracuse to take the Book but he discards the idea after meeting Proteus' fiancée, Marina. However, Eris later disguises herself as Sinbad and knocks out a guard before stealing the book herself. Syracuse is immediately plunges into darkness and Sinbad is arrested. When he states that he doesn't have the Book, he is sentenced to death until Proteus interferes, having believed Sinbad's earlier claim of Eris framing him. Sinbad is sent to steal the Book back from Eris and return it to Syracuse in ten days' time, or else Proteus will be executed in Sinbad's place.
Though initially reluctant, Sinbad agrees to confront Eris after Marina sneaks on board his ship and convinces him to save his friend. Meanwhile, Eris watches over him and sends many minions to intercept him, such as sirens and the gargantuan Roc, but Sinbad manages to evade these threats with Marina's aid. Eventually, Sinbad and Marina enter Eris' realm of chaos where Eris reveals her plan to them. She had known that Proteus would take Sinbad's place and that Sinbad, being a selfish thief, would sail away to paradise once he was free and leave his friend to die; this would then leave Syracuse without the next rightful king and gradually crumble into chaos. Eris hadn't counted on Marina convincing Sinbad to actually confront her, but she again plays on his supposedly selfish nature by asking him a question and promising to give him the Book of Peace if he answers it truthfully. The question is whether Sinbad would actually go back to Syracuse to die instead of Proteus if he didn't obtain the Book, to which he says yes. Sensing he is lying, Eris sends Sinbad and Marina out of her realm with an evil laugh, keeping the Book of Peace for herself.
Sinbad, knowing that he tried his best, has a change of heart and decides to sail back to Syracuse to be executed. As the axe is about to fall, it shatters into hundreds of fragments and an enlarged and livid Eris appears before him. Sinbad realises that he has now told the truth and Eris is forced to hand the Book of Peace over to him. Despite having been outwitted by a mere human thief, Eris quickly shrugs off her anger and tells Sinbad she is off to create more chaos elsewhere, before vanishing in a puff of purple smoke.
Role in the Spongebob series
(Role in the series to be desided later when I can find a fitting role.)