The Bard's Tale Wiki

A masterkey is a key that unlocks all doors. In the Bard's Tale games, however, doors are usually not of that make that can withstand a party of motivated adventurers. Only sometimes a well-placed kick against old wood won't do, when facing a veritable iron gate, for example. It is no replacement for the Onyx Key to Mangar's Tower either.

According to Roscoe, the masterkey also magically negates the effect of spinners on the party when equipped.

Tales of the Unknown[]

The Masterkey is an item that is sold by the Keymaster in Mangar's Tower. This is disappointingly late in the game, because by then the party is likely to have acquired the Onyx Key and does not need to go back to Kylearan's Tower.

The Masterkey can be used to open Skara Brae's locked iron gates that shut off Kylearan's Tower and Mangar's Tower.