The final key is a specialty item in Dragon Quest III. It allows the Hero's party to open all 3 types of doors which exist in the game (thief, magic, and final). It is the last key to be acquired in Dragon Quest III.
Types of Doors
The final key opens all the types of doors in Dragon Quest III. This includes thief doors (yellow, plain), magic doors (silver, ornate), and final doors (grey, prison-like). As the final key opens all 3 types of doors, it renders the other key types obsolete.
Acquiring the final key is one of the first major tasks the player must accomplish after receiving the ship in Portoga. It requires visiting atleast 3 separate locations.
The castle of Eginbear is guarded so that outsiders may not enter by normal means. By using and invisibility herb, the hero's party can sneak by the sentries guarding the entrance. Inside, there is a boulder puzzle located on the second floor which must be solved. The player must push three boulders around water hazards into a straight line on the north wall. After successfully completing the puzzle, the hero is rewarded with the Vase of Drought.
North Central Sea and Final Key Shrine
In the north central sea (just north of where the 2 continent "arms" cross each other), there is a small grouping of shoals in a 'T' pattern. By guiding the ship adjacent to the south center shoal, the hero can use the Vase of Drought to raise the Final Key Shrine. Inside the shrine, the final key is located inside a chest.