Somerset was created when the ghost of Aten created the sphere and Phobos then used his life energy to create Somerset from Aten's Sphere.
Deep within the core of the earth, after Cortez was defeated by Vulcan, Somerset lives on in the Underworld, soon to return to resurrect Cortez and destroy Vulcan for good.
Somerset's final defeat forced Phobos to morph him into a human, feeling betrayed, he found Vulcan and joined him as his partner, he secretly used the genetic labs to recreate a graviton generator, which caused a black hole, disaster struck, the cataclysm caused a deity of the past to appear, sealed within a mask, it had no name, it just needed a host body to cling to, Somerset was the one ofcourse, so it possesed him, much like Majora's Mask possesed Skull Kid.
