On Dec. 1, 1973, Black Sabbath unleashed their fifth, and final, universally adored masterpiece, Sabbath Bloody Sabbath. Universally adored by loyal fans, that is, since rock critics hated it as a matter of course -- just as they had every Sabbath record before it.

For Sabbath members Ozzy Osbourne, Tony Iommii, Geezer Butler and Bill Ward, recording the follow-up to 1972’s Vol. 4 was anything but a cakewalk, since drug abuse and hard partying had by then begun messing with their focus, and the sheer fatigue associated with four years of non-stop touring had also taken its toll.

The band’s initial attempts to write and record in the sunny but toxic environs of Los Angeles, in the summer of 1973, proved fruitless, so they hastily retreated to their more familiarly gray-skied English homeland and rented out a Gothic medieval castle in Gloucestershire. Their hope being that working in the castle’s dungeon would spark the inspiration needed to conjure up a new spate of occult-laced doom metal along the same, brutally fancy-free lines of what had come before.

But Black Sabbath had other plans, and just as soon as Iommi broke through his writer’s block by crafting the mighty riff powering Sabbath Bloody Sabbath’s signature title track, the new album’s creative process began gathering serious steam and moving in unprecedented directions, to boot, marked by increased experimentation and sophistication.

Even familiarly bruising heavy metal wonders like "A National Acrobat," "Looking for Today" and "Killing Yourself to Live" were possessed of a newfound confident maturity, thought-provoking lyrics and increased melodic sensibilities, while "Sabbra Cadabra," "Who Are You?" and "Spiral Architect" incorporated both synthesizers (courtesy of Yes legend Rick Wakeman) and orchestral arrangements to great effect. And in "Fluff," Iommi’s typically “throwaway” acoustic interludes of previous LPs found sweet, baroque redemption.

In other words, Sabbath Bloody Sabbath wasn’t just another Black Sabbath album, but a clear dividing line in their career; a significant creative leap inaugurating phase two of the band’s career and promising much of albums still to come. But, as history showed, ever-increasing substance use and continued exploitation by label and management taskmasters would duly exhaust Sabbath’s artistic powers and undermine the quality of subsequent albums to varying degrees.

By the time Osbourne was ousted from the group five years later, Sabbath Bloody Sabbath was already being recognized as perhaps the final great stand of the original, founding foursome.