After defeating the Eyegore, the chest containing the Giant's Mask will appear. It allows Link to grow significantly in size by wearing the mask in the boss fight with Twinmold, which is the only place where Link can use the mask. While giant in size, Link cannot use any items apart from his sword and shield, which includes the use of his special Spin Attack as he is already using his magic store by wearing the mask. It drains the Magic Meter for the duration of wearing it, with Link removing the mask to return back to normal size if it is depleted.
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