>> (WARNING SPOILERS AHEAD)Xex is responsible for all the monster attackes being done to Aselia. In other words, he's the main antagonist of the story.
Xex is actually an android from Magitechnology made by Dr. Garrel of Alvanista. The king believed that Garrel was starting an opposition, and was about to use Xex as a weapon to go against the empire. Because of this, Garrel was killed. Xex, outraged at the irrational acts of the humans, uses his powers to control monsters, and orders them to eradicate the human race.