Demonoid: Messenger of Death (1981)

Up from the depths of hell comes the ultimate horror!

A British woman visits her husband at the Mexican mine he is attempting to reopen and discovers that the workers refuse to enter the mine, fearing an ancient curse. The couple enter the mine to prove there is no danger and inadvertently release a demon which possesses people's left hands and forces them to behave in a suitably diabolical manner.