Their master/possibly creator is resonant with late afternoon gathering menacing gloom, hence the 'dusk' portion, and I expect they are prone to hunting things down for the 'hound' part.

We have now encountered three and killed two (the first one encountered the party when we were much frailer and one-hit KO'ed Celyn; it was, however, a partially scripted encounter and it was not able to finish the job).