Demon lords, also known as Abyssal lords, were archfiends who gained great power and established a position of preeminence among demonkind. Each demon lord had a unique appearance and set of abilities. Most controlled at least one layer of the Abyss. A demon lord, when slain in a plane other than the Abyss, was banished to the Abyss for a hundred years, whereas ordinary demons, including the mariliths and glabrezus, risked being reborn into a lesser form, or not all. A demon lord who had complete control over an entire layer of the abyss was known as a demon prince. Demon lord and demon prince were self-proclaimed titles; unlike the archdevils, the chaotic evil demon lords did not have a rigid hierarchy.

Demon lords waged eternal war with each other, often seeking ways to expand their holdings into other layers and at the same time defend their personal domains from rival lords and scheming underlings. Demogorgon had, for many years, been the most powerful demon lord, and proclaimed himself Prince of Demons – a coveted status symbol since the Age before Ages granted by the Abyss itself to the most powerful, ruthless, cunning demon in the plane. Miska the Wolf Spider was the first ot hold it, followed by Obox-ob, and now Demogorgon. Orcus and Graz’zt contested this title, while Obox-ob desires to regain it.

Most demon lords had secretive mortal cults who sought to elevate their lords to godhood through prayer and sacrifice.

source:http://forgottenrealms.wikia.com/wiki/Demon_lord