Well, I'll just make things explicit.
Possession (Su): Once per round, Ambuchar Devayam can attempt to merge his incorporeal body with a creature on the Material Plane. This ability functions as the magic jar spell, except that it does not require a receptacle and as outlined below. To use this ability, Ambuchar Devayam attempts to move into the creature's space; if the creature fails a DC X Will save, Ambuchar Devayam merges with the creature and takes possession of it. If the save succeeds, Ambuchar Devayam does not possess the creature and cannot move into its space, and the creature is immune to Ambuchar Devayam's possession for 1 hour. Attempting to possess a creature does not provoke attacks of opportunity.
While possessing a creature, Ambuchar Devayam keeps his Su and Sp abilities, including his gaze and touch attacks. In addition, each day that Ambuchar Devayam possesses a creature, the possessed creature suffers a negative level. These negative levels cannot be removed in any way while the creature is possessed, but they also do not result in actual level loss until the possession ends. However, as normal, a creature with as many negative levels as HD still dies.