From Monkey Island wiki
A locket is an important item in Tales of Monkey Island, being both of sentimental value and a powerful voodoo item.
The Voodoo Lady gives the locket to Guybrush Threepwood when she sends him to find La Esponja Grande to cure the Pox of LeChuck. The locket opens to reveal two pictures, one of the Voodoo Lady some years ago and the other of a handsome young adventurer. She tells Guybrush that the man is Coronado De Cava, her old boyfriend who is also looking for La Esponja. The locket is a token that will win De Cava's help in finding the voodoo artifact. It also acts as a key to opening a singing manatee decoration in De Cava's old shack on Roe Island, revealing a clue to finding the Cranky Fish summoning artifact.
Guybrush gives the locket to De Cava when they meet in the mouth of a giant manatee. De Cava pushes a little button that Guybrush never noticed and is briefly possessed by the Voodoo Lady, who gives important instructions to Guybrush. Guybrush later makes use of this power to possess the Voodoo Lady's body so that he can acquire the Marquis De Singe's book on the manatee language without having to set foot on Flotsam Island.
Coronado is enraged when he finds that Guybrush has placed his own picture over top of De Cava's (to enable the possession) and believes Guybrush is trying to steal his girlfriend from him. He and his crew attack Guybrush but he manages to call upon his friend the manatee to swallow their ship. The locket falls to the bottom of the ocean.
After Guybrush returns from the Crossroads of the afterlife, he finds the locket and is able to cause it to float to the surface, where Reginald Van Winslow finds it and places it on his raft. Guybrush can now use it (with Winslow's help to press the button) to possess his own dead body, which, when he makes it permanent, turns him into a zombie.
- In the PS3 version, if the player has Guybrush fiddle around with the locket combinations before possessing the Voodoo Lady in Chapter 3, the reward is the "Lord of the Locket" silver trophy in the "PlayStation Network Trophy Achievements". In Chapter 5, once Guybrush manages to possess his own body via the locket with help from Winslow and the Spirit Gum, the reward is the "I Ain't Got No Body" silver trophy (a reference to the song "I Ain't Got Nobody").