This is the song that became the name of the band. They were playing clubs in Germany and using the name "Earth" when they realized another band had the same name. "Black Sabbath" was lifted from the title of a 1963 horror movie starring Boris Karloff that was directed by the Italian filmmaker Mario Bava.
The group's lead singer Ozzy Osbourne and bass player Geezer Butler had seen the film, and decided to write a song with that title. When it became clear that the band needed a new moniker, they named themselves after this song.
The name change coincided with a new sound and image for the group. They had been playing blues (mostly covers), but started writing more original material and found a darker, heavier sound that defined them throughout their Hall of Fame career. Eschewing anything resembling R&B or psychedelia, they found a fan base hungry for something fiendish and new. Critics derided the band, but they quickly became one of the most popular and enduring acts of their time.
What is this that stands before me?
Figure in black which points at me
Turn around quick, and start to run
Find out I'm the chosen one
Big black shape with eyes of fire
Telling people their desire
Satan's sitting there, he's smiling
Watches those flames get higher and higher
Oh no, no, please god help me!
Is it the end, my friend?
Satan's coming 'round the bend
People running 'cause they're scared
The people better go and beware
No, no, please, no
Songwriters: John Osbourne / Terence Butler / Tony Iommi / W.T. Ward