The Jailor's Key is a key item in [[Dragon Quest II: Luminaries of the Legendary Line]]. It can be purchased at the item shop in Burrowell. It' is the precursor to the Ultimate Key used in later games.
The key can be purchased at the item shop in Wellgarth by selecting the blank line within the shopkeeper's list of available items. The fact that the shop sells the key is hinted by a townsperson in Wellgarth. Unlike the other keys, the Jailor's Key can be purchased multiple times and sold at shops like any normal item.
The Jailor's Key is required for accessing several key areas late in the game. In particular it is used to access Lagos' cell, the watergate in Slewse, and the travel gate leading to Rendarak in Beran.