Mask of Truth
From Zelda Legends Wiki
OverallGossip Stones that are scattered throughout OoT-era Hyrule and the lands of Termina. As visible from the picture the Mask represents the symbol of the Sheikah, which can also been seen on Sheik's clothes and the aforementioned Gossip Stones.
Ocarina of Time
Link can also obtain it in MM by completing the Swamp Spider House and returning the cursed man into his former self again.
Swamp Spider House's Man after breaking the curse:
You see, a while back, someone told me I could become rich and he gave me this mask.
He said the instructions were inscribed somewhere in here, but when I went to look, I was cursed! If I had known it would be such an ordeal, I never would have taken it. Here! Take it! It's yours!
The Happy Mask Salesman after you show him the Mask of Truth:
That is the Mask of Truth, isn't it?
You have a frightful mask. But being able to see into people's hearts and minds seems useful...