Buckthorn will wait to see the results of wizards ritual. Should further rituals fail to reveal any further details, Buckthorn suggests the party first take an extended rest and then start digging in the middle of the hill, under the face, where the sand and gravel are. "The tomb indicated the entrace was there," he offers.
EDIT: Seeing Forge start to dig, Buckthorn will join him. "Though we may want still consider an extended rest if we had not already, before we do something like, oh, open a door to some buried, brooding,foreboding tomb on the Shadow Coast..."