The second movie in the Tensei Shitara Slime Datta Ken series.

Fresh off celebrating Tempest’s founding festival, Rimuru and his companions are summoned to the Sorcerous Dynasty of Thalion, a nation of elves, by its influential empress, Elmesia. But their well-earned rest at her private resort island is cut short when a mysterious girl named Yura shows up, kicking off a new adventure on the endless azure sea..

(Source: Crunchyroll)