Summary
Thalassion is the giant-fish boss linked to the Backwaters fishing chain and the Gilded Rod upgrade.
How the fight goes
This is a good reminder that some bosses sit behind side systems. The route matters as much as the fight itself.
Attack patterns
Expect a fish-specific encounter with movement pressure that feels different from the island's land bosses.
Spoils of the fight
Defeating Thalassion ties directly into the fishing progression and the final rod upgrade.