Aernus, the Sleeping One, was a Progenitor Perennial originally worshipped by the Atlantean religion during the First Era, and later by many Perennial cults during the Second Era. One of the Sacred Twelve, Aernus was regarded as the embodiment of onness, transcendence and immanence, three qualities present in God. In the Aenean Church, he is regarded as the effective leader of the Sacred Twelve, and as the most powerful personality of God.
In the Indo-Persian mythology, Aernus is fully replaced by Ayavan, his role in the Atlantean and Aernean mythologies being given to Ayavan and altered to a great extent, where he is seen as both creator and destroyer. However, he and Ayavan are regarded as separate entities in Atlantean and Aernean mythology.