Mask of Truth

The Mask of Truth is an item in and. Its distinctive design prominently features a Sheikah Eye Symbol.

Ocarina of Time
In, the Mask of Truth can be borrowed from the Happy Mask Shop after completing the Mask Trading Sequence. Wearing the mask allows Link to speak to Gossip Stones to learn new things about the world and its characters. The Happy Mask Salesman mentions that the mask was passed down by the Sheikah.

If Link wears the Mask of Truth in the Forest Stage of the Lost Woods, he will receive a Deku Nut upgrade. A glitch in the Nintendo 64 and Virtual Console releases renders the upgrade unobtainable after getting the Poacher's Saw.

Majora's Mask
In, the Mask of Truth is given to Link by the Cursed Man in the Swamp Spider House. According to him, he was told that he could become rich if he were to look in the Spider House, and was given the mask. However he became cursed as he went inside, and was turned into a Skulltula-like creature. He tasks Link with defeating the 30 Gold Skulltulas in the house to lift the curse, and rewards Link with the Mask of Truth as thanks.

The mask has several uses. Wearing the mask allows Link to speak with Gossip Stones. Many Gossip Stones give clues on how to find other masks. The mask also imbues the wearer the ability to read animals' minds. This is mainly used in the Doggy Race to see which dog might win. Notably, the item description for this mask is the only direct reference to the Sheikah made in the game.

Theory
The Mask of Truth is speculated to be part of a set of three objects, including the Lens of Truth and the Stone of Agony, that were created by the Sheikah tribe to give their holder untold power in terms of perception of the truth of their surroundings.

Trivia

 * Paradoxically, the Gossip Stones in Termina will give Link clues on how to obtain the mask; however, the clues are only accessible if he already has the mask.