Tanar'ri in the Forgotten Realms represent the classical demons of human folklore. They are large, horned, bat-winged monsters cloaked in fire and shadow. All tanar'ri are Chaotic Evil and are native to the Abyss, an ashen plane that resembles some interpretations of Hell. Demons are rarely seen in the Prime Material Plane, but are sometimes seen gated in by powerful wizards, or summoned through use of vile rituals. Tanar'ri have an ongoing feud with their rivals in the Nine Hells (Baator), the Baatezu. It is called the Blood War. This hatred is so powerful Tanar'ri and Baatezu will prioritize killing each other in battle. Killing any tanar'ri that has appeared in the Prime Material Plane sends their essence to another realm - usually, their home in the Abyss, to be freely gated in again. Because these demons cause headaches for deities, many gods encourage imprisoning them in objects or dungeons on the Prime Material Plane to keep them out of trouble, in spite of the risks for primes.
In Baldur's Gate, the nabassu are the lowest ranked tanar'ri seen, while balors are the strongest.
Aec'Letec - an unusually powerful nabassu, worshipped by a regional cult
Yxunomei - a marilith commander of a demon army in Icewind Dale
Tahazzar - the balor commander of a group of tanar'ri trapped in Watcher's Keep
Demogorgon - Prince of Demons, imprisoned in the lowest level of Watcher's Keep