Mask of Truth

The Mask of Truth is a mask featured in both Ocarina of Time and Majora's Mask. Its distinctive design prominently features a Sheikah Eye Symbol. The stone is a companion relic of two other items: the Lens of Truth and the Stone of Agony. It is speculated to be part of a set of three objects, including the Lens of Truth and the Mask of Truth, that were created by the Sheikah tribe to give their holder untold power in terms of perception of the truth of their surroundings.

Ocarina of Time
The mask is obtained in Ocarina of Time by selling all of the sellable masks to various people all over Hyrule from the Happy Mask Shop. In this game, the mask has the power to allow Link to speak to Gossip Stones to learn new things about the world and its characters.

If Link wears the Mask of Truth at the Forest Stage in the Lost Woods before withdrawing the Master Sword from its pedestal for the first time, he will receive a Deku Nut upgrade.

Majora's Mask
In Majora's Mask, the mask imbues the wearer with several abilities, one of which is the ability to be able to read animals' minds, although this isn't a particularly useful skill except in the Doggy Race to see which dog might win. The second ability is the Ocarina of Time ability to talk to Gossip Stones.

Obtaining the Mask
The Swamp Spider House must simply be completed, which is located in Woodfall, to be rewarded with the Mask of Truth. Note that this requires Magic Beans, at least one bottle, the Sonata of Awakening, and the bow to complete.