Volume III of the original Arduin Grimoire and Arduin Trilogy.
From the inside back cover:
"These three volumes are in themselves a complete and playable game system."
However, what most people used them for was a supplement to the original D&D as it covered many aspects of D&D that were lacking or missing altogether.
Either way, it was a colorful FRPG and was well regarded by many in the lat 1970s before the advent of AD&D.