From Y! Entertainment:
Between 1970 and 1972, Black Sabbath released four albums that still stand as one of the greatest hot streaks in rock – Black Sabbath, Paranoid, Master Of Reality and Vol. 4. But when it came to making their fifth album, they found the tank was empty.
An attempt to write new songs in Los Angeles ended in failure. Panicked, the band returned to the UK and rented Clearwell Castle, in the Forest Of Dean, where things swiftly became spooky. “We were in the dungeon playing away, and all of a sudden we saw this person walk past the door [with] a big black cloak on,” said Geezer Butler.