Ronom is a creation of Monor 'Argendor' Felnor's mind. He is most assumably Monor's split personality.
He introduces himself as an otherwordly spirit, that settled down inside Monor's body. He has less control over the body than Monor himself, although he can make Monor do about anything he wants just by suggesting it to Monor.
(Everything in this page is Under construction)