In the Fellowship of the Ring, when Tom Bombadill is telling the hobbits stories of the nearby lands, he says:
"... Gold was piled on the biers of dead kings and queens; and mounds covered them, and the stone doors were shut; and the grass grew over all. Sheep walked for a while biting the grass, but soon the hills were empty again. A shadow came out of dark places far away, and the bones were stirred in the mounds. Barrow-wights walked in the hollow places with a clink of rings on cold fingers, and gold chains in the wind."
What is the shadow that came out of dark places and caused the barrow-wights to wake? Some websites point to the Witch King of Angmar, but I wanted to know if there are any canon sources to that. I always had the impresssion that barrow-wights are a totally separate evil and have nothing to do with Mordor. Like the Balrog and Shelob, I assumed they were another source of evil in middle earth.
When did the barrow-wights first wake and who woke them?