Xarxes is the god of secret knowledge and ancestry. He began as a scribe to Auri-El, and keeps track of all Aldmeri accomplishments whether they be large or small, since the beginning of time. Xarxes created his wife, Oghma, from his favorite moments in history. He is the author of the Oghma Infinium. 
In The Elder Scrolls V: Skyrim, the Daedric Prince Hermaeus Mora mentions that he revealed the secrets of the Oghma Infinium to Xarxes. In dialogue with his new (although possibly reluctant) champion, the Last Dragonborn, he calls Xarxes his "faithful servant" upon the discovery of the book. This essentially proves that Hermaeus Mora collaborated with Xarxes on the creation of the omnipontent lexicon as well as supposedly converting him from his service to Auri-El to the darker inscriptions of the Demon of Knowledge.