La maschera del demonio/Black Sunday

In Moldavia, in the year 1630, beautiful witch Asa Vajda (Barbara Steele) and her paramour Javuto (Arturo Dominici) are sentenced to death for sorcery by Asa's brother. Before being burned at the stake, Asa vows revenge and puts a curse on her brother's descendants. A metal mask with sharp spikes on the inside is placed over the witch's face and hammered into her flesh.
Two centuries later, Dr. Thomas Kruvajan (Andrea Checchi) and his assistant Dr. Andre Gorobec (John Richardson), are traveling through Moldavia en route to a medical conference when one of the wheels of their carriage is broken, requiring immediate repair. While waiting for their coachman to fix it, the two wander into a nearby ancient crypt and discover Asa's tomb. Observing her death mask through a glass panel, Kruvajan breaks the panel (and the cross above it) by accident while striking a bat. He then removes Asa's death mask revealing a partially preserved corpse that is visible underneath, her face staring out malevolently. He cuts his hand on the broken glass. Some of his blood drips onto Asa's dead face.
Returning outside, Kruvajan and Gorobec meet Katia (also played by Steele). She tells them that she lives with her father, Prince Vajda (Garrani), and brother Constantine (Enrico Oliveiri) in a nearby castle that the villagers all believe is haunted. Gorobec is instantly smitten by the beautiful young woman. The two men then leave her and drive to an inn.
The witch Asa is brought back to life by Kruvajan's blood. She contacts Javuto telepathically and orders him to rise from his grave. He does so and goes to Prince Vajda's castle, where Vajda holds up a crucifix to ward the reanimated corpse away. However, Vajda is so terrified by the visit that he becomes paralyzed with fear. Katia and Constantin send a servant to fetch Dr. Kruvajan, but the servant is killed before he can reach the inn. It is the evil Javuto who arrives to bring Kruvajan to the castle. Javuto leads Kruvajan to Asa's crypt, and Kruvajan watches in horror as her coffin explodes. From its ruins, the vampire-witch offers him eternal life (and a night of pleasure) and drinks his blood. By Asa's command, Kruvajan enters Vajda's room and murders him.
Asa's plan is to drain Katia of her blood, believing that this act will grant her immortality. A little girl who had seen Javuto meet Kruvajan at the inn describes the dead man to Gorobec. A priest recognizes the description as being that of Javuto. The priest and Gorobec go to Javuto's grave and find Kruvajan's body inside the coffin. Realizing that he is a vampire, they kill the fiend immediately by marking him with the sign of the cross and ramming a small piece of wood through one of his eye sockets.
Javuto finds Katia and takes her to Asa. Asa attempts to drink her blood but is thwarted by the crucifix around her neck. Gorobec enters the crypt to save Katia but finds Asa instead. Asa pretends to be Katia and tells Gorobec that the now weakened and unconscious Katia is really the vampire. She tells him to kill Katia immediately by staking her. He agrees but at the last possible moment he notices the crucifix she is wearing. He turns to Asa and opens her robe, revealing a fleshless skeletal frame. The priest then arrives with numerous torch-carrying villagers, and they burn Asa to death. Katia awakens from her stupor, her life and beauty restored.

Question: Karuvajan watches in horror as what explodes?
Answer: coffin