Giant's Mask

The Giant's Mask is one of the masks found in Majora's Mask. When equipped, the wearer becomes a giant, able to fight enemies many times their size. In Majora's Mask, it's only useable during boss battles, and even then it's only truly useful during the Twinmold battle in the Stone Tower Temple. Wearing the mask will drain Link's Magic Meter as well.

Obtaining the Mask
The mask is obtained in the Stone Tower Temple after a mandatory fight against an Eyegore.

Glitches
It is possible to put on the Giant's Mask elsewhere, using the same glitch used to wear the Fierce Deity Mask. However, Link will simply either yell as if he's transforming, and the game will lock up, or else revert to normal.