I am in no way an expert but there are some bits and pieces scattered across several works. Now I haven't read it myself, except for the free preview, but I understand that the PDF "The Tongue of Those Who Journey Beyond" has some interesting stuff in it.
http://rpg.drivethrustuff.com/product/1 ... Sunuz?it=1Otherwise what I think I know is in general the following.
1. There are three Pariah Deities; the Goddes of Pale Bones, the One Other and the One Who Is and each of them seem to have a goal for the universe that is less than productive; eating it, reducing it to singularity or its total enslavement
2. Their worship seems to be as old or potentially older than even the Tlokiriqáluyal/Lords of Change. The goddess of the Pale Bones was for instance well established when the First Imperium was created.
3. The Goddess and the One Other was relatively popular at times in present day Five Empires but have since been purged in an act of persecution never seen before or after. Some remains of the cults are thought to linger and outside the Five Empires its very possible that there are people who worshipp the Pariah Gods.