After The Never Say Die album and the band's tenth anniversary the following year, Ozzy was fired due to substance abuse - deemed excessive even by Sabbath standards at the time.
As luck would have it, Ronnie James Dio was looking for a new band after Ritchie Blackmore decided to steer Rainbow into commercial waters, and deemed Ronnie's love of sword and sorcery-themed lyrics surplus to requirements.
The diminutive singer's first jam with Sabbath went very well indeed, spawning the song Children Of The Sea that would become a favourite on the first album they would make, 1980's Heaven And Hell.
A departure of sorts for Sabbath, the album regained the ground lost on the lacklustre Never Say Die. Dio's powerful and theatrical voice inspired Iommi to write a record of frequently upbeat songs like Wishing Well and Walk Away, balanced by the epic title track and Children Of The Sea.
It was the beginning of a fiery, on-off relationship between the egos of singer and band that would take years - and two reunions - to stabilise.