After the tour-de-force duo of 1971's Master Of Reality and 1972's Vol 4, Sabbath hit a wall of writer's block for the follow-up a year later.

They decamped to Clearwell Castle in the Forest Of Dean in search of inspiration - a mock gothic mansion that would later also be used by the likes of Deep Purple and Led Zeppelin for writing and rehearsal.

With Sabbath set up in the dungeons, inspiration flowed into lush instrumentation and progressive territory. The album they wrote - Sabbath Bloody Sabbath - is the closest thing to their Sgt Pepper. Just ask Tom Morello and Billy Corgan about its enduring influence.

However, they had an unwelcome visitor during their time at Clearwell, as Iommi explained to Guitarist magazine in 2009:

"A cloaked figure [was] coming towards us in the hallway. We were setting up the gear in the dungeons and we were the only people there. It was myself and Geezer... or myself and Ozzy, we were walking down the hallway and we saw somebody coming towards us, we thought, 'Who is that?'

"It walked into this room and we followed it to see who it was and there was nobody there. It was an armoury with all the weapons on the wall and there was nothing else in there. We told the people about it who owned the castle, we thought they'd think we were mad. But they just said, 'Oh yes, that's the castle ghost'."