Approximately 19-20 years before the start of Dark Dawn, Ayuthay's Alchemy Well was restarted by a mysterious Adept. Once the well was started, it was discovered that the Luna Mask was needed to completely control the Alchemy Well. The mysterious Adept was able to locate the Luna Mask and installed in the Alchemy Well. The result that was the desert land of Ayuthay become an oasis, as the Alchemy Well produced much needed water for the city.
It is never stated outright where the Luna Mask was hidden. However, there are multiple images of the Luna Mask in Barai Temple, similar to the images of the Sol Mask in the Ouroboros. Additionally, there are empty treasure chests in the early parts of the dungeon, the lattermost of which was placed on a pedestal beneath an image of the Luna Mask, similar to the image of the Sol Mask in the Ouroboros placed above the chest containing the mask itself.
Luna is the Latin word for Moon.