The actual disk of the Sun. Originally an aspect of Ra. Gone a bit power-hungry. Wants all the worship for himself.
Religion around Aten is very focused on the daytime (obviously). Night is a time to be feared. Aten is supposedly a god who cares about all the creatures of the world. According to the religious principles, Aten created all countries and people. Aten also gives life but only to the ruling party (conveniently, Akhenaten and Nefertiti) who would then give life based on loyalty to Aten.