EX Treasure: Ancient Mask

"EX Treasure: Ancient Mask" is a Side Quest in.

Overview
The Quest is obtained after purchasing The Master Trials DLC pack and upon reaching Hyrule Kingdom. The Quest will activate upon loading the overworld of Hyrule, which can be done by Traveling, reloading a save file, dying, or leaving an Ancient Shrine. When the Quest is activated, Link will be tasked with finding Misko's EX Journal, which is rumored to be in the Outpost Ruins. In the Journal are clues to the locations of eight Treasure Chests, each containing a priceless relic stolen from Hyrule Castle by Misko. The Journal states that the ancient mask are at the ruins where soldiers gathered and the waters of Lake Kolomo can be seen. It is possible to activate the second stage of the Quest by reading Misko's EX Journal without triggering any of the loading conditions for the Quest's activation. The Treasure Chest can be found buried next to the southeastern corner of the largest building at the Kolomo Garrison Ruins. The Treasure Chest can be pulled out of the ground using the Magnesis Rune. Once free, Link can open the Chest to receive the Majora's Mask, completing the Quest.