Spoiler warning: Plot or ending details follow.

"The royal family of Hyrule collected historically significant outfits, but they were stolen by a bandit named Misko. One of the items stolen was an ancient mask. There's a journel that places the location of this treasure somewhere in the Outpost Ruins..."
— Adventure Log

EX Treasure: Ancient Mask is a side quest from The Legend of Zelda: Breath of the Wild. It is unlocked upon installation of the DLC Pack "The Master Trials". It requests Link to find and read the Misko's EX Journal at the Outpost Ruins. The quest guides Link toward the "ancient mask" treasure, a new Treasure Chest containing the Majora's Mask. According to the journal, the mask lies at the ruins where soldiers gathered near where the waters of Lake Kolomo are visible which refers to the Kolomo Garrison Ruins. Link can get the chest buried in the largest building of the Kolomo Garrison Ruins by using the Magnesis Rune.

Spoiler warning: Spoilers end here.

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