Grymkin
Overview - the history and background of the Wicked Harvest
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 sweet brand of whoop-ass on anyone with a shred of wickedness in their hearts, mainly by using nightmarish creatures that is just straight out of your worst fears or finding ways to punish you for your own cowardice or greed.
Surprisingly there is a delicious element of irony to the release of the nightmarish Gyrmkin upon the world as they may be its best chance for survival. The Old Witch's motive for unleashing these bowel-churning horrors was oddly enough so that the Grymkin could slow the spread of infernalism within the Iron Kingdoms that may one day end up bringing about a true hell on earth if left unchecked. By seeking out and killing those evil enough to deal with the Infernals and by toughing up good men through the crucible of the Gyrmkin's judgment it is the Witch's hope the spread can be stopped or at least muted enough that if the Infernals did invade the world would have a fighting chance of stopping them. 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. Told you the irony was sweet.
Considering the fact the Infernals have been announced as the next limited release faction for Hordes due out in Summer 2019 it seems the Old Witch's plan has failed sadly. Time to get out the holy water and salt...
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 then.