In the earliest of days, before the Valar entered Arda, Sauron was originally known as Mairon the Admirable, a powerful Maia of Aule the Smith, a Vala. However, Mairon was soon corrupted by the Dark Lord Morgoth an evil Vala and Dark Enemy of Arda, and turned evil, taking the name, “Sauron.” At first, he was a spy for Morgoth, telling him the Valar’s doings.
Then Sauron was ashamed, and he was unwilling to return in humiliation and to receive from the Valar a sentence of long servitude...for under Morgoth his power had been great. Therefore when Eonwe departed he hid himself in middle earth, and he fell back into evil, for the bonds that Morgoth had laid upon him were very strong.
THE MAIAR: Aiwendil. A Maia of Yavanna; had a special affinity for animals. One of the five Istari sent to Middle Earth to contest the will of Sauron. Also known as Radagast the Brown. Art by Igraciano.