The D'Arsay were an ancient humanoid race who originated in the D'Arsay system 100 million years ago. (Last Unicorn RPG module: All Our Yesterdays: The Time Travel Sourcebook; ST reference: Star Trek Encyclopedia))
The archive was eventually discovered at the center of a rogue comet by the USS Enterprise-D in 2370. After infiltrating the Enterprise's computer and accessing the ship's replicator technology, the archive began to transform the ship into the setting for a mythological battle between Masaka, the D'Arsay's sun goddess, and Korgano, the moon god.
The archive altered the positronic brain of Lieutenant Commander Data so that he took on the many different personalities of the D'Arsay people, including the personality of Masaka, who was attempting to transform the Enterprise into her temple. The situation was eventually resolved when Captain Picard took on the personality of Korgano, and the Enterprise returned to its natural state. (TNG episode: "Masks")
In 2409, more archaeological evidence was found relating to the D'Arsay culture. Gravestones from their people were found on planet Theta Caeli 670 in the Beta Quadrant, revealing clues about their biology. (STO video game: Star Trek Online)
The D'Arsay were humanoid in nature. They were apparently a poikliothermic species, which means their internal temperature varied along with their surroundings. This could very well explain why their sun and moon played such a large role in their culture, as their personality may have been altered with each passing season. (STO video game: Star Trek Online)