Death Smiles on a Murderer (1973) on Netflix

Greta, a mysterious woman with amnesia, is taken in by the wealthy Mr. and Mrs. von Ravensbruck. The three soon enter into a love triangle that turns deadly, as Greta's gruesome past is slowly revealed.

A young woman, Greta, loses her memory following an accident with a carriage and finds hospitality at the villa of two wealthy landowners Eva and Walter von Ravensbruck. The two welcome the girl with great hospitality--far too much--and she ends up in bed with the husband first, and then with the wife. Even Dr. Sturges, who pays her first visit, is interested in her and, above all, he is interested in the amulet that she wears around her neck. In fact, he is carrying out mysterious experiments to bring the dead back to life through ancient formulas attributed to the Incas and that amulet is very familiar to him. Things get complicated when Eva discovers Greta's affair with her husband and, in a raptus of jealousy, Eva kills her by entombing her alive. But a month later, during a masquerade party, Greta mysteriously reappears with a decayed face and begins to persecute Eva, pushing her to suicide - and the deaths will not end here.