Grymkin
The dim past
Ages ago, so long in the past that God-Emperor knows when it was, Menoth, God of Man, decided it was high time that humanity stopped sucking and actually did something useful for once, i.e. doing what Menoth ordered without complaint and telling him constantly what a cool dude he is. Basically he created a whole bunch of laws that men had to live by; they were given no choice in the matter unless they wanted to end up being tied to a pole and set alight for daring to think they had anything like free will.
Naturally not everyone agreed with that and five individuals with tremendous will of their own rejected him so strongly and fundamentally they ended up tapping into some part of the divine power all humans have in their souls and going over 9000! instantly in terms of badassness. Unfortunately Menoth was not too best pleased with these 'upstarts' and threw them all into Ur-Caen (the afterlife) while still alive. Now trapped in this spirit world these Defiers were endlessly tormented by their own nightmares made real for thousands of years, an endless agony that quite understandably lead them to swear revenge against Menoth and his twisted creation.
Despite their imprisonment the five Defiers had become demi-gods and in time they learned to control their divine power. Through careful study the Heretic was able to learn how the Defiers could bend their own nightmares to their will, bringing them under their control and giving them access to an army that would make you crap yourself if you saw them coming at you. It was not just their nightmares that would become the Defiers' underlings, for those wicked souls who end up in the wilds of Urcaen were judged and transformed by the Defiers as an eternal punishment into forms suited to their crimes, cursed to hunt down those with evil in their souls. This growing gathering of vengeful demi-gods, nightmares given flesh and damned souls would in time become known as the Grymkin, dedicated to unleashing judgment on a cruel and corrupt world.
For centuries, the Defiers awaited their chance to return to Caen to unleash their wicked harvest on Menoth's civilization (and anyone else that needs a good kicking, a certain dragon springs to mind..). The Old Witch Zevanna Agha knew of the Gyrmkin and looked for a way to set them free to further her own plans. She managed to find a lady in an insane asylum and used her as a conduit to negotiate terms (who knew an old hag was so well versed in contract law?) and arrange to open the gates to Urcaen for them. Now the Gyrmkin are ready to unleash their judged
It was not until the Old Witch devised a way to create a portal to Urcaen through which the armies of the grymkin could pass that they got their chance. The Old Witch's motive for unleashing hell on earth was--oddly enough--to use the Grymkin to slow the spread of infernalism that would one day unleash an even worse hell on earth if left unchecked.
If Infernals ever invaded Caen in full force, it would mean the end of the world and the eternal torment of all living souls. The Grymkin, then, are the world's twisted salvation from annihilation.
Hilariously the only thing the Grymkin warlocks (Defiers) are truly scared of is their warbeasts which are in truth their own nightmares. I'm guessing obedience school didn't work and they will