Greyshire Glacial Grotto, known just as Greyshire Grotto on its map and in the Japanese version, is a dungeon in Final Fantasy XV that becomes accessible 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.
Completing the dungeon and retrieving the royal arm is part of the game's main quest. The player receives the quest from Talcott Hester in Lestallum. The player can travel to this location and retrieve the royal arm before talking to him, and if so, he will tell them about the waterfall and the party will tell Talcott they already retrieved the weapon.
"Greyshire" is a Japanese play on words. The Japanese katakana rendering for the English terms "grey" (グレイ, gurei?) and "shire" (シャー, shā?) compound to make グレイシャー (gureishā?), the Japanese rendering for the English term "glacier". This play on words is less obvious in English, so the word "Glacial" was added to its name in in-game text to preserve the second meaning.
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: the Swords of the Wanderer.