The Mirror of Truth is an item that appears in The Water Temple story scenario from Hyrule Warriors and Hyrule Warriors Legends. According to legend, the Mirror was hidden within the Water Temple by the Hero (it is unclear if it was left their by the Hero of Time or The Hero responsible for sealing away the four fragments of Spirit of Evil's Soul).
It is found in the Water Temple by Princess Ruto who becomes trap by the Dark Forces inside the keep where she finds it, forcing her allies Sheik, Impa, and Darunia to lower the water level in order to reach the Gatekeeper so they can defeat him and open up the keep. After the gatekeeper is defeated, Princess Ruto freed and hands the it over to her allies and retreats to safety. It dispels the illusion around the Hall of Mirrors Keep where the Era of Hero of Time's Gate of Souls is located and is used to reveal that Wizzro is trying to defeat the heroes under the guise of Princess Zelda.
In the english localization of Hyrule Warriors and Hyrule Warriors Legends, it is known as the Lens of Truth.