The Majora’s Mask is one of several new items introduced in The Legend of Zelda: Breath of the Wild‘s first DLC package, titled The Master Trials.
This item can be be found at the Kolomo Garrison Ruins, just East of the Coliseum Ruins next to Lake Kolomo. You can get here very early on in the game following its tutorial.
Once at the Kolomo Garrison Ruins, look for a treasure chest that is cemented into the ground. Use your Magnesis power to pull it out, and then open it to retrieve the Majora’s Mask.
This item makes enemies think you’re friendly, allowing you to sneak by them. It’s a great option for newer players who want to avoid difficult encounters, or veteran players who are looking to move quickly through areas.