From Lexicanum

The Treachery of Vision is a text written by the Eldar Farseer Eldrad Ulthran of Craftworld Ulthwé.

This series of writings spoke of the beginning of existence and how the universe had formed. Within these texts, Eldrad also spoke of the Old Ones as well as their war with the Ancient Enemy which had emerged from the hearts of a thousand sons. As such, it spoke of the time when the C'tan were given physical form and how they waged a war against the Old Ones. In addition, it spoke of how the Old Ones drew power from the raw Immaterium to combat this foe.

According to "The Treachery of Vision", Eldrad postulated that it was during that ancient conflict between the C'tan and Old Ones which led to the formation of Daemons. He also mentioned that he had heard that there were Eldar more ancient that Asurmen that were born during this time and fed on the power of the warp drawn by the Old Ones. This text also spoke of the Black Library and how the most ancient of times are being guarded by the Harlequins which bear an imprint of the Old Ones themselves. Eldrad wrote that he had seen those texts and noted that they were exquisite.

Eldrad Ulthran spoke of how if the Ancient Enemy returned, than their return would signal the completion of their Great Work which would seal the warp itself and that the sons of Asuryan would be cut off from their life source. Furthermore, the Eye of Isha would dim and close thus ending the great legacy of the Old Ones. This is because the Ancient Enemy possess no souls and thus have little to fear from the severing of reality. The Farseer also wrote of how the Eldars races desire brought the manifestation of Daemons onto the galaxy.¹

Source

• ¹:Dawn of War Omnibus Ascension by C. S. Goto (pg 279-281)