off-topic + urgent: unknown french thriller desparately searched

Best adressed to all french users, but as well to aynbody else whom it may concern:
I search the title of a french thriller I´ve seen in the mid-eighties which was propably shot in the early eighties:
It´s about a banquett in the cellar of a mansion. All participators of the baquett were poisoned by the host and just left where they sat, then the vault was sealed by him.
The special thing is, that in the vault there is a huge artwork hanging at the wall, showing the banquette with each guest but with the heads of ancient egyptian gods instead of their own.
Only the host -on the artwork the hosting god Osiris I guess- (the killer) is missing and that is thereby "Hint 1".
The movie begins with the police opening the vault (solid steel gate), discovering the crime that has already done monthes or years ago. Thereby the released stench of the bodies makes them rush out to throw up. The killing spree that was started with the banquett ain´t over, but is continued which is the story of the film.
Btw it´s kinda the script they propably would offer Jean today to take the leading part as the inspector.
Does somebody know this movie?
Merci beaucoup!
And GOOD LUCK for the World-Cup-Game over here tonight!!!
Allez les bleus!