Baalzebul

Formerly an archon known as Triel, was the lord of Maladomini and most infamous archdevil next to Asmodeus himself. He was known as the Lord of the Flies because his webs of intrigue were so tightly woven that not even flies could escape, but the form he was cursed with by Asmodeus earned him a new, hated title; The Slug Archduke. Eventually he managed to overcome this wretched state and return to his previous glory, and if only to retain it, had never told a lie to his kin since. Triel was once renowned as among one of the most beauteous denizens of Celestia but was physically altered after his corruption. His new form was that of a 12 ft  tall humanoid with shimmering, sable skin and the jittering, compound eyes of a fly. Still angelically stunning even after his fall from grace, Asmodeus put a curse upon him as punishment for a later crime, forcing him into an even more demeaning form. The former celestial's mutated form was that of a 25 ft tall slug, a grotesque abomination infamously unpleasant even to other devils. The Lord of the Flies was constantly surrounded by swarms of the buzzing, scuttling creatures that earned him his title. His body glistened with mucus and thanks to his curse, fecal matter and other rancid filth manifested in piles wherever he went, resulting in his notoriously noxious stench. His deformed arms rested at his front and were more comparable to prehensile tails in functionality. Replacing his handsome visage was a gross, human-like face with torpid lips, incapable of properly pronouncing words without a slimy, wet lisp. However, even when reduced to such an unsightly state, faint glimpses of the Slug Archduke's previously glorious appearance could still be made out. He had recently managed to work off his punishment, partially freeing him from Asmodeus's curse and allowing him to return to his humanoid form.
Of Baalzebul's numerous flaws, his most crippling was his perfectionist extremism. His unfiltered dedication to achieving physical flawlessness and greater dominion was what caused his initial fall from the Seven Heavens, as such callous ambition was more befitting of a devil. Seemingly incapable of realizing when he had gone too far, his utter arrogance led him to undermine his own plans by overreaching. He might have even been able to defeat his rival Mephistopheles if in his overconfident scheming he didn't also aim to defeat Asmodeus, causing him to fall even further than before. Baalzebul made an effort to seem calm on the surface, often withholding his true feelings from others. In truth, the Lord of the Lies had viewed his transformation into a slug not simply as an obstacle to be overcome but as a humiliating experience. He had become an abusive taskmaster during his time in Hell, a sadistic torturer that displaced his anger on his subjects and loved disheartening them, but after being cursed he became neglectful and miserable himself. Often times he ignored his depressing duties in favor of plotting revenge and fantasizing about regaining his original, Triel form. Despite his flaws, Baalzebul was still dangerously cunning and charismatic. If not for the interference of Geryon, he might have not only been able to best Mephistopheles, but even oust Asmodeus himself. Even when trapped in his inarticulate, slug-like form, he was still the Lord of the Lies, whose every deception and false statement was made with ease. He was not to be mistaken as a pathological liar as he told each untruth with a purpose in mind and indeed managed to keep a hold on his layer for several millennia without lying, at least to other devils.
Various spell-like abilities were at Baalzebul's command, such as the power to manipulate and create light and flames, create powerful undead, enchant and mentally command others, turn invisible and use various types of divination magic. His more powerful abilities let him create symbols of pain and insanity, speak unholy words and even use wish. He could choose to summon barbazus, erinyes, gelugons or pit fiends but particularly cornugons. Normally however, Baalzebul's spells were his fallback plan used when his other supernatural powers, proved unsuccessful at dealing with his enemies. When in his slug form, Baalzebul's first tactic was normally to belch out gargantuan clouds of flesh-devouring flies, an ability he had to wait a few dozen seconds to reuse. Although the flies normally dispersed after being spewed, he could cause them to swarm and envelop his form if he so desired for about a minute, protecting him from harm as the flies didn't attack his grotesque mass. The flies could be blown away by a strong gust of wind, but even without them shielding him, Baalzebul's stench was so putrid that simply trying to get anywhere close to him was sickening. Despite being more sluggish than he once was, he was able to burrow underground and his adhesive slime-coated form was adept at scaling surfaces.
His arms were so feeble that he normally relied on his servitors to perform tasks like turning pages, opening doors and feeding him, but they served a purpose in combat besides wielding weapons. Blood, muscle and sinew quickly withered away with even the slightest touch from the Slug Archduke's atrophied limbs, although immortals like devils or demons were immune to the effect. Like an actual slug however, limbs and body parts removed from Baalzebul simply grew back, although his regeneration was supernaturally fast, allowing him to regrow body parts in a few minutes and reattach severed limbs. His regenerative powers were less effective when he was harmed with good-aligned spells and holy weapons.
Although many of his powers were a result of his slug form, Baalzebul possessed some before his metamorphosis, such as the ability to fly or shapechange. Another was his direct gaze, which combined the powers of a fear and ray of enfeeblement spell to leave his victims trembling in terror even after he stopped staring at them. His slug form shared his gaze, but couldn't use it properly when surrounded by flies. After he was restored to his humanoid form, Baalzebul retained his ability to shapechange. He was capable of transforming into a giant fly or a swam of fist-sized flies, similar to those he could previously spew.
Several magical artifacts of great power could be found within Baalzebul's domain but, presuming one could avoid his cornugon guardians, were practically impossible to find amidst the collapsed structures and random refuse. The Book of Vile Darkness once made its way into his personal library, and of the six full copies known to exist, at least one contained extra pages added by the Lord of the Flies.
After his mortifying experience as a slug, Baalzebul limited his open plotting to the Lord of the Eighth and focused his energies on sneaking more spies into Asmodeus's court while also appealing to him, primarily by making his rivals look incompetent by comparison. The curses inflicted on him by his master effectively neutralized his ability to properly forge alliances with his peers, so much of his time was spent completing his actual duties as an archduke and overseeing the soul-driven bureaucracy of his layer. Edicts, policies, treatises and other documents were kept in the underground archival labyrinth that Maladomini had made after his defeat, protected by traps, devils and confounding classification systems. He jealously guarded all the power and secrets he could from the other archdevils, particularly Asmodeus and Mephistopheles, in order to eventually enact his take over of Hell, and at one point hoarded souls with intent to perform a ritual to escape his sluggish form.
Baalzebul had several rivals in the Nine Hells and enough enemies that at some point he was given the insulting corruption of his sobriquet, "Lord of the Lies", although in some cases it was still used as a compliment. His notable foes were Mephistopheles and Dispater, both of which were ancient archdevils whose seniority contrasted Baalzebul's quick upstart and who scorned the one they viewed as an outsider. Both were pleased to hear of his transformation into the Slug Archduke and during his transformed period Baalzebul drove his spy network to discover, or at least plant, evidence of their wrongdoing in the hopes of getting them trapped in even more demeaning forms or, if lucky, completely destroyed. Although he feigned subservience, Baalzebul's primary target of hatred was Asmodeus for cursing him, yet the Lord of Nessus occasionally favored him for seemingly no reason, possibly viewing him as a worthy opponent. Despite Baalzebul keeping them in his castles and having them as servants and bodyguards, he only begrudgingly allowed pit fiends into Maladomini, suspecting them of being puppets or spies for Asmodeus.
Incapable of admitting his own fault, Baalzebul placed the majority of blame for his curse on Mephistopheles. The Cold Lord was the greater of his rivals and the two could be said to be quite similar, particularly in their insatiable hunger for power and recognition. Both believed that the key to their success was to defeat the other, making them the second greatest archdevil and strong enough to potentially overthrow Asmodeus. Each spent much of their time either machinating against their archenemy or having their forces engage in skirmishes and battles in a constant stalemate, which ironically was part of the reason Asmodeus tolerated them.
A keeper of many secrets, Dispater's hatred for Baalzebul was the most well-known in Baator as the two constantly fought wars of intrigue. Even with the support of Mephistopheles, the forces of Dispater and Baalzebul slowly whittled down each other and protecting himself from such plots appeared to consume much of Dispater's time and distract him from further plans of expansion. Part of the reason behind his utter loathing of the Fallen One was that he viewed him not as a true baatezu but just an exiled celestial undeserving of the respect he showed Mephistopheles. More recently however, Dispater sought to politely distance himself from his allies and make peaceful gestures to his foes, however impossible being a truly neutral party in the Nine Hells was.
One of the curses inflicted by Asmodeus upon Baalzebul was that any deal struck with him would inevitably end in catastrophe for the other party, a decree that generally warded off other devils from doing so. Despite this, Baalzebul still had his allies, such as the Lord of the Fourth Belial, who had been with him both before and after the Reckoning. Their relationship was kept private after Baalzebul was demoted but both sides would come to the other's aid if they were in danger, although in Baalzebul's case that was based on the assumption he could find some benefit in doing so.
Of all the archdevils, Baalzebul was second only to Asmodeus in terms of infamy and was as popular as any other deity in Faerun, not that his cultists made their worship public knowledge. Even after his transformation, his cult was larger than those of his peers although most mortals were unaware of the curse that doomed those who made deals with him. Whether as the Lord of the Flies, Lord of the Lies or the Fallen One, he was despised as much as he was revered, luring in mortal followers as the dual patron of sly charm supported by brutal power. Being paragons of violent cunning, bugbears were his most common followers, with the heart of Baalzebul's army being a team of sixteen fiendish bugbears called the Nyashk. Each was a sorcereress armed with powerful spells that wore a helmet resembling a fly's head and wielded a harrow rod. Baalzebul specialized in mass-scale corruption and deception, with his religious orders functioning more as conspiracies against previous establishments. His sects were often composed of nonconformists, rebels and other malcontents and focused on disrupting through a combination of charismatic guile and terror campaigns involving assassination. He tried to instigated coups against the leaders of lawful empires in order to place imperial dictatorships in their stead, but as a result of his subtle methods it could take his cults generations to actually succeed. When such plots actually did come to fruition, Baalzebul's servitors normally proved more adept at subverting authorities than acting as them. Because of this, Asmodeus, who normally targeted an already existing authority rather than secret societies, occasionally swapped temples with Baalzebul, with Asmodeus taking over Baalzebul's finished projects and Baalzebul being granted access to hidden shrines to Asmodeus in just kingdoms.
After regaining his old form, Baalzebul positioned himself as a savior of failures, preying on individuals believing themselves to be in need of redemption, such as disgraced or destitute nobility. Whether due to disloyalty or incompetence, he and his agents found those that had lost important resources or honor and offered supernatural solutions to their problems. Of course, even without the curse of failure, secret clauses, fine print and inconspicuous specifications were hidden throughout contracts made with the Lord of the Lies, all designed to ensnare the unwary and desperate. Despite harvesting a great number of such individuals, souls able to be lured in such a manner were typically so pathetic or inept in the first place that they were only useful as wretched cannon fodder. Those formally dedicated to the Fallen One were treacherous and ruthless, willing to use whatever deceitful tactic and betray even their closest friends and family to achieve their aims. They could summon swarms of flies but specialized in using cunning and eloquence to tell bold-faced lies. Allies were rarely kept around along before being backstabbed although his disciples excelled at manipulating them so long as they were required. Even after he set himself up as a patron of redemption, his followers strove to regain respect at the cost of those who caused the initial loss, and did so by tripping up allies in order to improve their relative standing.

"Through sufficient penance and grace, even the lowliest can redeem themselves. Am I not a living testament to that fact?” — Baalzebul
“Your gods have fled this place. The spirits of the world no longer walk your streets. Your mages’ spells have failed you. In your hour of need, I alone hear your pleas.” — Baalzebul