So what were the creatures? *SPOILERS!*
Judging from what I've seen from this movie and what I've read of H.P. Lovecraft's original story, the creatures in this film seem to be humans from the Martense family that are products of incest, feeding off of corpses, and living underground for a long, long time. That kind of background, along with their looks and how strong and tough they are, leads me to believe that they are ghouls or a prototype for ghouls. What do you think they are?
Welcome to my Nightmare- Freddy Krueger