Link is first transformed into a Deku Scrub after he follows Skull Kid into Termina, who curses him into this image. The curse is lifted and the mask is acquired upon learning the "Song of Healing" from the Happy Mask Salesman. In order to achieve this, Link must first retrieve the Ocarina of Time that was stolen from him by the Skull Kid. The song causes the Deku spirit residing in Link to be encased within a mask, which then falls at his feet. The Happy Mask Salesman then tells Link to keep the mask as a memento. With the mask, Link can freely transform into a Deku Scrub at will.
The Spirit Within the Mask
The Deku Butler and his son
During his travel through the Catacombs, Link runs across a tree bearing resemblance to a Deku Scrub, which Tatl even points out as looking like Link. During the ending credits, the Deku Butler, who lost his son, is seen standing near the tree. It is possible that he was killed by Skull Kid, who then uses his spirit to transform Link.
In January 2001, Nintendo Power held a contest where the winners of the second prize would receive a copy of Majora's Mask and one of five ceramic replicas of the masks used in the game. These include the Deku and Zora Masks.
Deku Mask artwork from Majora's Mask
Deku Mask as seen in game from Majora's Mask
Link obtaining the Deku Mask
Nintendo Power Majora's Mask contest
Lana wearing the Deku Mask in Hyrule Warriors
↑"When you wear it, you will transform into the shape you just were." — Happy Mask Salesman (Majora's Mask)
↑"Then listen to me. Please play this song that I am about to perform, and remember it well... This is a melody that heals evil magic and troubled spirits, turning them into masks." — Happy Mask Salesman (Majora's Mask)
↑"Wait! That's it! Your instrument!!! The mask salesman said that if you got back the precious thing that was stolen from you, he could return you to normal! Did you completely forget or what?" — Tatl (Majora's Mask)
↑"Fear not, for the magic has been sealed inside the mask. When you wear it, you will transform into the shape you just were. When you remove it, you will return to normal." — Happy Mask Salesman (Majora's Mask)
↑"I give you this mask in commemoration of this day." — Happy Mask Salesman (Majora's Mask)
↑"It's strange, but the way you look right now sort of looks like this tree..." — Tatl (Majora's Mask)
↑"Actually, when I see you, I am reminded of my son who left home long ago..." — Deku Butler (Majora's Mask)