The Veil

Knowledge about the true nature of the Abyss/Thoon can have a corrupting effect. The Sundering brought home these dangers, and the gods swore not to share what they knew of the Abyss.
Unfortunately, some secrets still got out, especially among the Il'yana with their ties to Keryth and the Demons she used to rule over. This led to the Harrowing.
To make sure that the Harrowing could never happen again, the gods, particularly those with the Secrets Domain, placed a veil designed to shield the mortals from this discovery. Initially very effective, the Curse has produced cracks in this veil, and some individuals pierce the veil and learn some terrible truth about the Abyss. This process is dangerous to body and mind, and some don't survive it, resulting in the emergence of an Allip.