Abhumans can range from massive creatures, like the kretch, to relatively weak ones. The truth is, there is not much to tell about abhumans, apart from that they are the offspring of the Fiend, as they differ so much. For example, there were four abhumans pulling the fiends coffin who were 9 feet tall and had ram like horns.
The Making Of
They are made when the Fiend, and a witch are together. It is a way of keeping the Fiend at bay.
Abhumans are first seen in the Spook's Apprentice - and are seen throughout the Wardstone Chronicles.