Install The Master Trials DLC.
The Royal family found these fairy clothes that they have found. The Great bandit Masiko stole and hid the clothes somewhere in Hyrule.
- Hint: "The green garments of the man who wished to be a fairy are hidden in the ruins of Hyrule Field."
- Find these pieces at the ruins where trade flourished...
- at the ruins where sinners were imprisoned...
- and at the village ruins next to farmland where many people once gathered."
The royal family of Hyrule collected historically significant outfits, but they were stolen by a bandit named Misko. Among the items stolen were a fairy's green garments.
There's a journal that places the locations of the treasure somewhere in the Outpost Ruins.