Jan Jarmokowski is the villain of the short story There Shall Be No Darkness and the film The Beast Must Die. In the film, he is played by Michael Gambon.
Jan was a pianist of possibly European origin. He was also afflicted with the deadly curse of lycanthropy. He possessed no control over his transformations. Only by killing someone and eating their flesh could he return to human form. His curse destroyed his career as a concert pianist. Every time he played a concert in a country, there'd be grisly murders. The police suspected him, but could never prove anything. He eventually fled to England. The curse also threatened his relationship with his student turned lover Davina Gilmore, but she remained with him despite everything, never suspecting him of being a werewolf.
While living in England with Davina, the pair were two of five people invited to millionaire hunter Tom Newcliffe's country mansion. Jan knew Tom only in passing, as Davina had once roomed with Tom's wife Caroline. However, Tom knew all about Jan and the murders committed around him. As it turned out, Tom invited the five people because he suspected one of them of being a werewolf. In an effort to both escape, and to protect Davina and the others, Jan tried to escape in his Mercedes, but Tom pursued in a Land Rover. Tom's knowledge of the grounds of his estate allowed him to cut Jan off, whereupon he persuaded the piano player to return to the house.
Jan transformed on the first night. Tom hunted the werewolf (not knowing it was Jan) through the forest, but Jan doubled back to the mansion. He encountered and killed the Newcliffes' chief of security, Pavel, after which he returned to human form and went back to his bedroom. Tom concealed Pavel's death from the guests, and sabotaged all the cars. While in his human form, Jan made at least one attempt to kill Tom by hurling an axe at him, but he missed.
The second night, Jan again transformed and left the house. Tom pursued him in his helicopter and eventually cornered him inside a barn. In the ensuing battle, Jan fatally injured the Newcliffes' dog and also killed the pilot of the helicopter. His curse having advanced so far, one taste of human flesh wasn't enough to return Jan to human form, and on returning to the mansion he claimed his third victim in two days, Arthur Bennington. Only then was he able to become human again and pretend as though he'd been somewhere else.
Luck was with Jan. Tom suspected artist Paul Foote of being the werewolf and focused almost exclusively on him. Furthermore, while tending to the wounded dog, Caroline got the dog's infected blood in a cut on her hand, and became a werewolf herself. On the third and final night, she transformed, forcing her husband to shoot her and kill her. Afterward, Jan transformed one last time, killed Paul, and fled the house. Leaving a terrified Davina with Dr. Lundgren, Tom pursued and ultimately shot Jan, freeing him from his curse.