Mephistopheles

Mephistopheles, also known as the Lord of No Mercy and the Cold Lord, is the lord of Cania, the eighth of the Nine Hells. 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 that 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.

Background
Once the court magus of Asmodeus in Nessus, Mephistopheles gained the rulership of Cania by seizing it away from Rimmon. After the Reckoning, he seemed to fall into a slumber, remaining frozen and unmoving, until he led a coup against himself in the guise of Baron Molikroth and then destroyed all his co-conspirators against himself.

In more recent events, Mephistopheles had persuaded his half-breed son, Magadon, to release the devil inside of him and therefore bringing himself and his fellow companions Erevis Cale and Riven Drasek to Cania through some manipulation of Erevis Cale's ability to travel the shadows. Mephistopheles then proceeded to kill his half-breed son, when Erevis Cale stepped in and promised Mephistopheles a portion of the god Mask's divinity to him in return for Magadon's soul. Mephistopheles agreed to this under one condition, that condition being he kept half of Magadon's soul there until delivery. Erevis Cale later returned to Cania and sacrificed his life for the return of Magadon's soul. It was at this time that the new demigod Riven Drasek, who had also absorbed part of Mask's divinity traveled to Cania and paid Mephistopheles a visit. Riven attacked and badly wounded the archdevil, and told him that if he ever stepped out of his domain that Riven would utterly destroy him. Mephistopheles replied by telling Riven that he will come back, and he will be waiting for him. He seemed to think that Riven would be back for Erevis Cale at some point in time.

Mephistopheles, along with Dispater, was once an ally of Mammon, but Mammon betrayed them both.

Attack on Toril
Not too long after 1372 DR, a drow matron afterwards known only as the Valsharess managed to bind Mephistopheles 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.

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 ordering him to kill the adventurer broke the pact which she used to bind him. 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. He raised the dead from the recent battles between the Valsharess and her enemies as a special form of undead army and began to follow a similar course of conquest to hers, advancing from the Underdark to Waterdeep. 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 before he could destroy the City of Splendors.

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