Ent was just reading about the
Barrow-wights someplace, but can't recall exactly where.
That said, this from "Complete
Tolkien Companion" aptly summarizes what Ent took away from the reading...
"But after the final fall of the
North-kingdom,
evil spirits from faraway Angmar entered the mounds and animated the dead. These ‘Barrow-wights’ walked in the hollow
places among the stone-rings and the Downs became more than ever a place of fear to those of nearby lands."