Archfiend

An archfiend was a particularly powerful fiend, often a demon lord or an archdevil, and usually a creature that ruled over one of the fiendish planes or had a fiendish army at their disposal. There were nine archdukes of the Nine Hells, an unknown number of yugoloth rulers, and considering the infinite layers of the warring Abyss, possibly an infinite number of demon lords.

Demon Lords
Also called abyssal lords, demon lords were powerful demons that commanded many other demons and ruled at least one layer of the Abyss. Each demon lord had a unique appearance and set of powers and abilities. The most powerful demon lords were called demon princes. Since demons had no set hierarchy, both titles were self-proclaimed, and demon lords schemed and fought against each other constantly. Prominent demon lords included Demogorgon, Orcus, and Graz'zt.

Archdevils
The archdevils were the most powerful of the devils, and operated in a strict hierarchy. There were nine archdukes of Baator, each archduke ruling one of the nine layers. In addition, there were other archdevils, mostly former archdukes who had been deposed or banished. Asmodeus was the Lord of Nessus (the ninth layer of Baator), and was the most powerful archdevil.

Yugoloth Lords
The yugoloths had no set hierarchy or rulers, instead being ruled by, among others, the ultroloths and baernaloths, as well as various altraloths. The Oinoloth, in particular, was a yugoloth archfiend.