The Barrow-Wight: Servant of Melkor?

[Lord of Angmar]

I think it is basically implied that it is talking about Sauron as the Dark Lord, since the Witch King of Angmar is solely the servant of the aforementioned, and in all of his wars after he became possessed by one of the Nine, his primary goal was to advance the causes of Sauron. The barrow-wights, then, would seem to have the same focus in mind, and since it was during the time of Sauron's growing power that Tom Bombadil recited this passage about the barrow-wights to the four hobbits (who were on a quest to stop the Dark Lord Sauron), it is only logical to assume that Bombadil was referring to Sauron Gorthaur.