The Abominable Dr. Phibes Movie Wiki

. He has reserved the final punishment for Dr. Vesalius. He kidnaps the doctor's son and places him on a table on which a container full of acid is waiting to destroy the boy's face. A small key implanted near the boy's heart will free him, but Vesalius must perform the surgery within six minutes to get the key before the acid falls. Vesalius succeeds and instead Vulnavia is sprayed with the acid as the police arrive.
Convinced he has accomplished his vendetta, Phibes retreats to a stone sarcophagus beside the embalmed body of his wife. As he drains out his own blood and replaces it with embalming fluid the coffin's inlaid stone lid slides into place, concealing them both in darkness. Trout and the police arrive and discover that Phibes has mysteriously disappeared. Trout and Vesalius recall that the "final curse" was darkness and they speculate that they will encounter Phibes again.