The Mirage Mirror (Illusion Mirror in the fan translation) is a key item in Trials of Mana.
Kevin using the mirage mirror in the Jungle of Visions
If Kevin or Charlotte is your protagonist, you will find the Darkstone in the Jungle of Visions. After defeating Zable Fahr, the Benevodon will drop the Mirage Mirror. At the end of the path, you will find a dead end, but Faerie will get the idea to try the Mirage Mirror to reveal the path. Instead, Goremand is revealed, and the battle begins.
After Goremand's defeat, the Mirage Mirror is not used again.