The “EX Treasure: Twilight Relic’ side quest that The Master Trials DLC adds to Legend of Zelda: Breath of the Wild will let you find Midna’s Helmet from The Legend of Zelda: Twilight Princess. Wearing it will boost your resistance to Guardians and, frankly, it looks pretty darn cool.
Misko’s EX Journal: Outpost Ruins
The “EX Treasure: Twilight Relic” side quest for the helmet appears automatically the first time you start the game with the DLC installed. Its text reads:
The royal family of Hyrule collected historically significant outfits, but they were stolen by a bandit named Misko. One of the items stolen was a helmet of twilight…
There’s a journal that places the location of this treasure somewhere in the Outpost Ruins…
Head to Ja Baij shrine on the eastern edge of the Great Plateau. Turn directly northeast and paraglide down off the plateau. Continue northeast until you see the Outpost Ruins. Just on the other side of the first wall you see, you’ll find Misko’s EX Journal.
Reading Misko’s EX Journal will update your adventure log for all eight of the stolen relics — Majora’s Mask, the Tingle armor set, Midna’s Helmet and the Phantom armor set. For the Twilight Relic, Misko wrote:
The princess of twilight, whose stories are handed down alongside those of the Hero of Twilight…
Her helmet can be found at the temple ruins soaked in the waters of Regencia River.
How to find Midna’s Helmet: Sage Temple Ruins
The soaked ruins he’s talking about are north-northwest of the Outpost Ruins. If you check your map, you’ll find the Regencia River to the northwest. Follow it north until it meets the Tamio River, just east of the Jeddo Bridge. The Sage Temple Ruins that you’re looking for are on the southeast corner of this intersection.
The easiest way to get here is to hop over to Sheem Dagoze shrine. Head east and onto the Jeddo Bridge. About halfway across, you can turn left and jump off (or you can keep running and turn east after the bridge). Paraglide down along the river and turn a little to the right as you come out of the canyon. You should land right on the edge of the Sage Temple Ruins.
Look for two trees. Between them and a little in front of them, there’s a pile of rubble with a rusty shield and a rusty claymore embedded in it. Stand on the north side of this pile of rubble and fire up your magnesis. You’ll notice a treasure chest buried under the rubble.
Pull out the chest and open it to claim Midna’s Helmet.