The Hidden Key is the key that allows you to unlock the 
Rengoku sword, hidden in a black chest in the 
Ice Castle. It disappears if you die or leave the game.
How to get it
5% chance from the 
Awakened Ice Admiral, lower chance from 
Snow Lurkers and 
Arctic Warriors in the Ice Castle. The 2x Boss Drops gamepass doubles the chances.
How to Use
Open the black chest in the secret passage under the stairs of the Ice Castle to obtain Rengoku. The chest can only be used once and disappears afterward. Equipping Rengoku gives you the "Samurai" title.
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