The Sol Mask was the product of a joint partnership between the Exathi, an ancient race of skilled craftsmen, and the Jenei, ancestors of the Adepts. It can essentially be thought of as a control mechanism for the Alchemy Forge. Without it, the Alchemy Forge can be turned on, but its abilities cannot be used in any form. Most likely, the Sol Mask was created so that only their allies would be able to harness the Alchemy Forge. At some point, the Sol Mask was hidden deep in the Ouroboros labyrinth, upon which the modern city of Kaocho rests.
Matthew and his party would travel to Passaj in hopes of reviving the Alchemy Forge, as it was the key to a passageway over the Khiren Mountains. However, the Adepts soon discovered that the Alchemy Forge needed the Sol Mask to become fully functional. This was a major problem, as Kaocho, the city that lay on top of the ruins, was in a state of war with Passaj and Passaj's ally, Ayuthay. The Adepts would travel to Kaocho in hopes of finding the Sol Mask, but found that they could not find the true entrance. This led the Adepts to Ayuthay, where they would meet King Paithos and his nephew, the Mercury Adept, Amiti. Paithos revealed that the Insight Glass, a treasure of Ayuthay, would reveal the entrance to the Ouroboros. The Adepts would explore Barai Temple, where Amiti would become the wielder of the Insight Glass.
The Adepts, with Amiti in tow, would return to Kaocho, and were able to discover the secret entrance to the Ouroboros. They were able to traverse the labyrinth, find the Sol Mask, and return it to Passaj without Kaocho discovering them. Once the Sol Mask was installed in the Alchemy Forge, it became fully operable, transforming the small town into a large fortress.
The name of the Sol Mask comes from the word "sol," meaning "sun" in Latin, a reference to the Alchemy Forge's fire-based powers. Its counterpart is the Luna Mask, used for Ayuthay's water-based Alchemy Well.