Gibdo Mask

The Gibdo Mask will is given to Link by Pamela's father in Majora's Mask. The mask causes Gibdos and ReDeads to recognize Link as one of their own, and will not attack him. While wearing the mask, Link can in fact talk to Gibdos, and this is necessary in the Beneath the Well portion of the game. ReDeads cannot be talked to, but the mask will cause them to dance, implying ReDeads always dance when they deem themselves unwatched.

Obtaining the Mask
Upon entering the house in Ikana, Link finds Pamela's father in a state of half-mummification. By playing the Song of Healing, her father will be healed, and the mummified portion of him encased in a mask.

Trivia
The Gibdo Mask is one of six masks that are necessary to complete the game, the others being the three transformation masks, the Garo's Mask, and the Captain's Hat.