The fact that, while you know what's attacking the town, the fact that you don't see anything of who or what it is, save for some well-timed glimpses, and a very fast-moving, sentient fog until the very last few minutes of the movie makes the tension of the film even more intense.
A Fridge Horror example: After the titular fog rolls its way in and cuts power to the entire town near the climax, the last we see of our inhabitants save for the designated protagonists is them filing their way to the unveiling of the statue for their town's centennial celebration. They're never seen or heard from again from that point, even after everything's resolved. It's safe to assume that they made it out all right, but given how fast the fog moved through the town, and what we saw happen to the people who were exposed for too long to the fog, the implications are more than clear.
The ending...or to be more specific, right AFTER the ending, radio host Stevie Wayne has survived by the skin of her teeth, the other survivors gathered in the church have managed to wait out the fog and Father Malone seems to have lifted the curse on Antonio Bay by returning the treasure (in the form of a cross made out of the melted-down gold) to Blake. The movie comes to a close with Malone (who spent most of the film in disbelief at the dark secret behind Antonio Bay's founding) walking through the church and questioning aloud why Blake only took five lives and not six as the story implied... We see fog billow under the door again as Malone walks away, before, eerily silent, Malone turns and sees that the revenants have reappeared, motionless and waiting. In this case, for Blake, who himself has materialised behind Malone, to swing for his head.
Subverted as the original theatrical release doesn't cut as Blake's blade makes contact...instead we see Malone's slightly surprised face as he turns away and a spray of animated, bright red blood is drawn from his neck. Which somewhat removes the surreal, chilling intent of the above example.
