Deku Mask

The Deku Mask is the first mask acquired in Majora's Mask. It is one of several masks that contains the spirit of another being (in this case, a Deku Scrub.) Wearing the mask will cause Link to take on the form of the Deku Scrub, and gain new abilities.

Obtaining the Mask
The Deku Mask is a mandatory mask. The mask is obtained after using the Song of Time for the first time. After talking to the Happy Mask Salesman, he will teach you the Song of Healing, which will free you of the Deku spirit, encasing it in the mask.

The Spirit Within the Mask
During his travels through the inner Clock Tower, Link will run across a tree bearing a resemblence to a Deku Scrub, which Tatl even points out as looking like Link. During the ending credits cutscene, the Deku Butler (who lost his son ) is seen standing near the tree, implying that the tree is his son, most likely killed by Skull Kid, who then used that spirit to transform Link.