Armlet of Transmutation
The Armlet of Transmutation (Golden Bracelet in the NES version) is a critical plot item in Dragon Quest IV. It is a magical artifact essential for successfully performing the Secret of Evolution; without it, one's transformation will turn out imperfect and warped.
Alena and her party discover it in a cave during Chapter 2; however, perhaps not realising its importance, they bargain it away as ransom for the Fake Princess of Santohaim.