The party receives the key from the Queen of Femiscyra after they prove their innocence for stealing a nun's golden brooch.
Nera Briscotti's father asks the party to check on a pot on a nearby island. Soon after, Bjørn the Behemoose escapes from the pot, and upon his defeat, the ultimate key is obtained. It will unlock any door in the game.
It is found in the Gortress in a chest guarded by Goreham-Hogg. It will open any locked door in the game. However, the chest (along with the one to the right, containing Sterling's whistle), cannot be opened until after Goreham-Hogg's defeat. It is a required item, as it is the only key that can open the captive Celestrians' cells, and the only one to activate a cutscene upon acquisition. If the player has not yet gained possession of the other two keys (the Thief's Key being obtained via Cap'n Max Meddlin' and the Magic Key from a chest in the Mirage Mahal), the player can still obtain the Ultimate key.