Mephistopheles

Mephistopheles, also known as the Lord of No Mercy and the Cold Lord, is the lord of Cania, the eighth plane of Hell. He was the main opponent of Baalzebul during the Reckoning of Hell, and still holds a claim to his own layer. He seeks to take Baalzebul's layer away from him, that he may gain enough power to one day challenge Asmodeus for rulership of all the Nine Hells.

Description
Mephistopheles is a nine-foot-tall, red-skinned humanoid with handsome, yet diabolical features which include a set of bat-like wings. He has huge muscles that befit his great strength, and his speech is whispering wind. His wings, horns, and talons are deep red and his scales are sooty black. His eyes are white with red irises and pupils.

Attack on Toril
In or soon after 1372 DR, a drow matron afterwards known only as the Valsharess found out Mephistopheles's true name and used it to bind him into her service on the material plane, in Toril. Aided by his powers, she proceeded to conquer much of the Underdark, and even went on to try to expand her influence on the surface, attacking Waterdeep through Undermountain. Unable to resist the power of his true name, the archdevil bided his time until he was able to turn the rules concerning the use of true names against her.

Previously, an adventurer fleeing the briefly re-risen but again falling Netherese city of Undrentide into the Plane of Shadow had come across a strange relic that they used occasionally to access the small plane of a strange creature known as the Reaper. This item was in fact meant for the leader of a cult of Mephistopheles, and contained a piece of the devil's own flesh. Later the adventurer, by then quite famous and powerful, came to Waterdeep to aid the city in its struggle against the Valsharess's forces. Mephistopheles manipulated the Valsharess to eventually capture the adventurer into her inner sanctum, where she ordered him to kill them. At this point, her control over Mephistopheles was loosened, since the relic carried by the adventurer bound them to the archdevil, and a creature's true name cannot be thus used to order them to kill what is essentially a part of themselves. Instead of complying, Mephistopheles let the adventurer free to duel and kill the Valsharess. He then sent them to Cania in his stead and set out to conquer Toril, intending to turn it into a new layer of Hell that would go down below the Ninth and make him the new supreme ruler of the plane. However, the adventurer, who had by this time grown in power far beyond that of most other mortals, was able to escape Cania, return to Toril and banish Mephistopheles back to his old domain.

According to this account, Mephistopheles's true name is Tra'axfyl the Ambitious.