One of the early Heavy Metal bands:

[ProfessorGAC]

11. Sabbath Was Earlier, Oo

The first album came out in early 70.
Paranoid was released later that same year. Yeah, they released 2 albums in 9 or 10 months!
Then about a year later, they released Masters Of Reality.
Three critically important metal albums in about 19 months!
Disagreeing with the sarge a bit here, about Iron Butterfly. Definitely heavy, but a lot of elements of psychedelia to be metal, for me.
With the interplay between organ & guitar, they were almost prog metal before there was such a thing.