Need for an SS-Rank Ally

Eren had proposed a win-win deal to his brother.

Although the sea monster no longer needed Eren's help to destroy the Raashim Temple now that he was free, he could still allow Eren to take charge of his enemies' territory once he was done with them.

After all, Eren was not only his brother but also his benefactor. Thanks to the Immortal Artifact's influence, the sea monster naively felt that he owed Eren a thing or two.

Furthermore, the sea monster had no desire to rule the seven seas. The only reason he had attacked the Raashim Temple at the time was that he had not been in complete control of his actions.

In the past before his arrest, the sea monster's mind had been consumed by thoughts of survival as the Immortal Artifact had tried to destroy him from within for its own amusement. Even if he had won against the Raashim Temple back then, he wouldn't have ruled over the seas.