The Murder Mansion - 1972 - 6/10
AKA - La Mansion de la Niebla

You know, you can warn people over and over. If the door to a subterranean world opens, don’t. If you find catacombs, tunnels, turn around.
Six disparate travelers, driving the backroads, get lost in rising fog, seek refuge in an isolated manse.
Out on the woods, stalking one of them or more, are ghosts of the original homeowner and her chauffeur.
Giallo takes a lot of time showing characters, building tension.
Midway between Horror and Thriller, with a fair dollop of Scooby Doo.
Eventually gets going, and the conclusion is satisfying. My version seems PG.