The Blood Realm is a planar world where demons live. It's a twisted mirror image of the world of Aravorn. It's origin/discovery is tied to the birth of Bloodskins race.
 Origins (Bloodskins)
Centuries ago, there was a bloodline of powerful magicians. So powerful that they were able to bend the very fabric of the world and shape it according to their will. One day, though, they went too far. Seeking to open a gateway to the home of the gods, they accidentally opened a gateway directly into the opposite direction - the place we call the Realm of Blood.
It's a twisted mirror image of our world. There are kingdoms, and empires, and different races, just like here. But all of them are demons, higher and lesser ones, and they feast on the suffering that's created in our world.
The magicians that had accidentally made their way into the Realm of Blood were no match for the demons. They would have been held there and tortured for all eternity, feeding the demons. But instead, they were able to strike a deal. Each of them would promise fealty to a demon lord, and then return to this world to serve their dark purposes. And each of their children, and their children's children, and so on, would suffer from the same curse. Whatever they did would cause pain and suffering in this world, which in turn would feed their demon masters.
Of course, there would have been an easy way out: All the Bloodskins would have had to do was to retreat to somewhere secluded, stop having children, and die without ever having done anything for their masters.
To prevent that, the three most powerful demon lords bound the Bloodskins to their names, and their names were Decay, Desolation and Deceit. Those bound to Decay had the task of creating offspring, of being fathers and mothers to new children, all of which would be carrying the curse of one of the three lords. Those bound to Desolation had the task of seeking out the company of others, so that there were ties between the Bloodskins and the rest of the world. And those bound to Deceit were given the order never to leave a task uncompleted, never to stop something they had begun, so that they could never prevent the suffering they were causing, even if they saw it coming.