What lies beneath the Edge in the novel, The Weirdstone of Brisingamen?

Beneath the Edge are the forgotten shafts of dwarves that connect with the human mines and the surface in surprising ways. One ancient network of tunnels is overseen by Cadellin, a wizard guarding one hundred and forty silver-clad knights from the world's youth. Another is the home of the "svarts," a nasty tribe of goblins in uneasy alliance with the morthbrood, a coven of local witches.