When I was in my early teens (1980's) I was invited to a Horror film party and we watched a film that has been stuck in my mind ever since, but I can't remember the title and I would love to see it again. The film (in color) is about a group of people who stay in a mansion and are each killed one by one (and its not house on haunted hill). The mansion has many secret passages and tunnels and someone is the killer. One scene that I will always remember is when the killer (masked?) traps a woman against a passage wall and uses two large knives as scissors and cuts off her head with one motion...very vividly!!!

I thought this forum may stir somebody's horror film brain and identify the film for me...can anyone name the title???

thank you