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.

Mephistopheles is something of a walking contradiction. Unstable and thoroughly wicked, he presents multiple faces to those he meets. On the one hand, he can be charming, erudite, and civil. But beneath the veneer of respectability is a vicious temper and unchecked ambition. He is patient and cunning, yet when alone, he flies into a fury, screaming and shrieking, tearing at his skin and destroying everything around him in a thunderous explosion of hellfire and devastating magic. He has one goal, one reason for existence: He covets Asmodeus’s throne. It is, in his mind, his destiny to rule the Nine Hells. So confident is he that he has told this to Asmodeus himself. One would think that such arrogance would be reason enough for the Lord of the Ninth to bring this archdevil to heel, but curiously, Asmodeus hasn’t. It seems he’s content to let Mephistopheles have his delusions. Mephistopheles' naked ambition and haughtiness does not sit well with his peers. Some are drawn to his power, such as Dispater, but most despise him, seeing him as an unstable and unpredictable element in the Hells’ convoluted politics. Among his enemies, Baalzebul is his greatest. The Lord of Lies has long opposed his every effort. So long as Baalzebul lives, Mephistopheles spends his time hatching plots to eliminate his ancient rival, leaving Asmodeus secure in his position as the Lord of the Nine Hells. Despite his distractions, Mephistopheles still poses a grave threat. He commands legions of ice devils as well as the pit fiends, barbazu, and cornugons that have flocked to his banner. In addition, it falls to him to protect the only gate into Nessus, so he nominally commands the army whose sole duty it is to safeguard Asmodeus’s realm. To make matters worse, he has mastered the essence of Hell, channeling it into a foul destructive energy called hellfire. With such tools at his disposal, it seems Mephistopheles will one day make good on his promise to rule in Asmodeus’s stead. Thankfully, Mephistopheles does not exert the same influence on the Prime Material Plane. Many mortals confuse him and Asmodeus, believing they are one in the same. This frustrates Mephistopheles to no end; above all, he wants to be worshiped as a god. Still, he has a number of small cults that revere him as the god of hellfire. He also attracts disaffected devilworshipers, stealing individuals from other archdevils. His temples are strange places, hidden and out of the way. Decorated with fire pits, they are unbearably hot and the pits are rigged so that they flare up during ceremonies at the most dramatic moments. The cultists meet to perform sacrifices, binding their living victim on a blackened altar and immolating him alive. His screams add the chorus to their silent prayers.

Appearance
This diabolical, nine-foot-tall fiend has crimson skin and handsome-yet-diabolical features: white eyes with red irises and pupils, long, straight, black hair, broad leathery wings, curling horns, and sharp talons, all of the deepest red, and sooty black scales. He has huge muscles befitting his great strength, yet his speech is as whispering wind. Swathed in a flowing black cape, he is the vision of evil.

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 bring himself and his fellow companions Erevis Cale and Drasek Riven 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 Drasek Riven, 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 to history 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-activated but now again falling Netherese city of Undrentide into the Plane of Shadow had come across a strange relic that they had 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 bring 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 by ordering him to kill the adventurer she broke the pact which she used to bind him.

Instead of complying, Mephistopheles left the adventurer and his party 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 souls he had gained from the recent battles between the Valsharess and her enemies as a special form of spiritual 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 by learning the Reaper's true name (Hecugoth the Abandoned), 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 Thra'axfyl the Ambitious. It can be deduced however that since Mephistopheles remains a threat to the planes as nobody's slave, the adventurer either did not learn this or chose for some reason to not use it to it's full potential.