Psychomania

Psychomania (1973) is a British outlaw biker horror movie. It stars Nicky Henson as a devil-worshipping biker gang leader and Robert Hardy as the detective in charge of bringing him to justice.
Story.
Tom Latham is an teen psychopath, and the leader of a violent teen motorcycle gang. The gang dabbles in black magic. They call themselves "The Living Dead". Tom's mother and her butler Shadwell get their kicks holding séances. With their help, Tom makes a pact with the devil to return from the dead.
Tom and his biker buddies commit suicide one by one with the goal of returning as one of the "undead". Not all succeed, but the ones who do return gather together at a secret place called "The Seven Witches" (a circle of standing stones). They continue to terrorize the locals.