searching french thriller .............SOLVED

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 on german TV 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 banquette 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.
The cops even trace a potential victim and take him under protective custody putting him an officer to the side, but while the Inspector is on the way to pick them up, the man recieves a letter which´s envelope cuts him as he opens it so he dies venomed.

It´s kinda the script they propably would offer Jean Reno today to take the leading part as the inspector.
̴m 90% sure it was a french thriller.

The way the crime has been artfully arranged and the ruthlessly of it reminded me a lot about "Dr Phibes", so I wanted to try it over this board after doing the same on the boards of several french thrillers past monthes without result.

Does somebody know this movie?
thx