Greyshire Glacial Grotto

Greyshire Glacial Grotto is a dungeon in Final Fantasy XV and visiting it is part of the game's main quest in Chapter 3. It is an ice cavern hidden behind a waterfall west of Lestallum. The player can fight a Midgardsormr on the grounds outside it.

Story
After hearing rumors of a Royal Tomb being located nearby behind a waterfall, Prince Noctis's party heads out to investigate. They find there is an ice cavern behind the waterfall and scale its depths until they come to a locked door. Noctis has a key for it, and inside they find a Royal Tomb and Noctis retrieves another of his Royal Arms.