The Head of Janus

The Head of Janus German Der JanusKopf is a 1920 German horror silent film directed by F. W. Murnau. The film was an unauthorized adaptation of Robert Louis Stevensons novel The Strange Case of Dr. Jekyll and Mr. Hyde, but the source material went unrecognized by some of the German media due to changes in the characters names.

Conrad Veidt plays Dr. Warren the Dr. Jekyll character who changes into Mr. OConnor a parallel of Mr. Hyde. This transformation is brought about, not by experimentation with chemicals as in Stevensons original, but through the supernatural agency of a bust of Janus the Roman god of the doorway, which Warren OConnor purchases in the opening sequence as a gift for his sweetheart, Jane Lanyon Margarete Schlegel. When she refuses the gift, horrified, Warren OConnor is forced to keep the statuette himself.It is at this point Dr. Warren first transforms into the gruesome character Mr. OConnor, and returns to Janes house in a rage, kidnapping her and taking her back to his laboratory. Upon recovery, Warren is horrified by what he has done and tries to sell the bust at auction, but the hold it has over him forces him to buy it back again. A second transformation proves to be his ruin, committing random acts of violence in the streets. ........

Source: Wikipedia