They were explained in the black company though, something which I am sure that they were taken from
Spoilers
the black company had creatures called Shadows that literally snuck up and sucked the souls out of their victims immobolizing them as they did it, there was no deffense besides magic. They were souls that had gone through so much pain (died of torture after being tortured for 30 years or something else) that their pure essance of pain could never leave the mortal world, thus explaining their world. The pain could be emotional or physical.
Many of them were created by the Ten who were Taken or in our world the Fallen Lords. I mean they are mean and horrible and old... they'd have time.
