from Book of Vile Darkness
Like the demon princes, the archdevils are the greatest of their kind and wholly unique. Unlike their demonic counterparts, however, the archdevils are organized in a strict hierarchy. The archdevils rarely make open war against each other. Instead, they plot and scheme in intricate webs of politics, duplicity, and betrayal.
The archdevils are also called the lords of Hell, the Dukes (or Archdukes) of Hell, and the Lords of the Nine. Together they rule all the layers of the Nine Hells. Each archdevil has his or her own dark palace, legions of devils and monsters, and schemes to gain more power. Tre various archdevils continually make and break alliances with each other and other planar powers in their quest for greater might and control.
The greatest of all archdevils is Asmodeus, who rules over all of them from the bottom of the Pit itself. He has controlled the Hells for as long as histories have been recorded, although there are rumors of even older lords of the Pit, with names such as Satan and Lucifer. These beings, however, are long gone now.
Not terribly long ago (as things are measured in Hell, anyway), Asmodeus masterminded the Reckoning, when the face of hell changed fairly dramatically. The archdevils Baalzebul and Mephistopheles warred with each other for the rulership of hell. With the help of a lord named Geryon, Asmodeus convinced the armies of each side to turn against their masters. All of the other archdevils fell before the onslaught of their own troops. Asmodeus reinstated most of them - with the exception of Moloch, who was replaced by the Hag Countess, and Geryon, whose position was taken by Levistus. (Leave it to the master of all hell to betray the only archdevil that remained loyal to him.)
Currently Bel, Mammon, Levistus, and the Hag Countess have no real allegiances. But that could change. Politics in hell being what they are, most likely the devils will polarize again. One thing is for sure: There is more hostility, and more fear, toward Aimodeus than ever before.
With the exception of Bel, the archdevils do not concen themselves too much with the Blood War. Hell's generals are a council of pit fiends called the Dark Eight, mighty warrior that helped Asmodeus win the day during the Reckoning.
from Hellbound: the Blood War
With each of The Dark Eight in charge of a particular aspect of the Blood War, a curious body'd think something greater's in charge of them. If such a being exists, mortal sages don't know about it. What they do know about are the Lords of the Nine, the rulers of the layers of Baator.
The Lords sit as far above the Dark Eight in might and influence as the Dark Eight sit above a spinagon. They're not full-fledged deities, but their status is still out of reach of most baatezu. See, the Lords are the monarchs of Baator. They leave the running of the Blood War to the Dark Eight (letting the pit fiends direct the battle as they see fit), and instead focus on tending to the plane itself.
The chant says that each Lord presides over an entire layer of Baator. Their dominion stretches across the layer, and they're always trying to extend their reach to yet another part of the plane - into the territory of other Lords. Some say their power is absolute on their chosen layer, much like a deity's will is absolute in its realm. But the gods can resist the desires of the Lords; not even the rulers of Baator dare to cross the powers of the plane. Rather, the two groups co-exist, their ambitions sometimes colliding, sometimes coinciding.
A few graybeards figure that there's another strata of fiend in between the Lords and the Eight, a group of noble-class baatezu who serve the Lords in their endless machinations and schemes. Supposedly, the Lords use these puppets to play off each other and sow discontent among their enemies. Like the princes of a mortal court, the noble fiends dabble in intrigue and politics, but with prizes far greater than any dreamed of by petty human courtiers.
from Faces of Evil: the Fiends
Plenty of myths surround the Lords of the Nine. Some folks say they're archfiends, risen from the ranks below them to dominate the layers of Baator. Other graybeards claim they're really yugoloths (I swear, some folks see 'loths everywhere they look). There's even a group that thinks the lords grew up from the very essence of Baator itself and are the living manifestations of the plane's will.
Is any of it true? Who knows? The lords keep most of the facts about themselves dark; the less folks know of 'em, the better. Still, it doesn't prevent tenacious mortals from digging up whatever they can. Take Willgan the Dogged, for instance. That blood gathered together more material than nearly any other. 'Course, the baatezu say his sources were corrupt and Willgan ain't available foe questions - he's vanished.
Anyway, here's what we do know about the lords. They're mighty, maybe even mightier than the deities of Baator. They're partly creatures of symbology as well as physical form, so their natures change bit by bit over the millennia. They've hidden the truth about themselves behind countless walls, giving different names and showing different aspects to almost every berk who asks, so that no one can truly label them. Like powers, they can manifest avatars and choose their forms for best effect.
The lords shape the layers of Baator in their own image and police their territory better than the Harmonium ever could. They command handfuls of noble baatezu and armies of common baatezu, and they use their underlings' power to amass ever more influence for themselves. And though they're bitter rivals, they recognize the need for diplomacy and alliances now and again to keep Baator strong against outsiders. What else? Well, the lords don't give a fig about the Blood War, leaving it all in the hands of the Dark Eight. They simply rule the nine layers of Baator, They also make pacts with leatherheads on the Prime Material Plane, promising strength, wealth and sometimes immortality to those who follow their tenets. They build civilizations and crush empires. They are, in short, never to be messed with. That's all I can say with certainty. Anything else is just guesswork. 'Course, some guesses are better than others, and the chant below is a collection of the most compelling evidence ever found. But mind where you take this information, berk - flap your Bone-box too loudly in the wrong places and you'll wind up in the dead-book for sure.
from Manual of the Planes (3ed)
The center of the Bronze Citadel holds the personal fortress of Bel, a pit fiend of great power who controls Avernus; in addition to being Lord of the First. Bel is also a general in the Blood War, though not one of the Dark Eight. Bel betrayed the former Lord of the First, Zariel, for his current position. It is whispered by his subordinates that Bel still keeps Zariel prisoner somewhere deep in the Bronze Citadel so that he can siphon her hellish power into himself, increasing his own abilities while slowly reducing her to just another soul shell. While Bel has succeeded where many failed before him, his climb to power seems to be stalled now, since he does not enjoy the support of other Lords of the Nine, except for possibly Asmodeus.
Unlike with Bel, it is difficult to class Dispater as a specific “type” of devil or recall his origin, other than to say he is an archdevil. Like the other Lords of the Nine, he is an elite devil whose powers are waxing, and even pit fiends fall before his onslaught. He usually appears as a humanoid of supernatural height, dark hair, tiny horns, and resplendent garb. He always holds his badge of office, a rod of singular power. Dispater never takes risks and rarely leaves his Iron Tower. Only Asmodeus's call brings him forth. In the politics of The Nine Hells, Dispater and Mephistopheles, Lord of the Eighth, are allies, usually scheming against Baalzebul, Lord of the Seventh.
Mammon is an archdevil and so possesses almost deity-level power. In form, he resembles a 30-foot-long serpent with a human torso, head, and arms. He wields a harpoon-like spear, though its magic is the least of Mammon's impressive abilities. He rules from the center of the city of Minauros, inhabiting a mausoleum like structure of similarly massive proportions. Mammon is a turncoat's turncoat. Though he once allied with Dispater and Mephistopheles against Asmodeus, he was the first to abase himself before the Lord of the Ninth when Asmodeus finally proved victorious at the close of a failed rebellion called the Reckoning. Few of the other Lords would ever trust Mammon again, making him least likely to be involved in the next internal revolt.
Belial and Fierna are archdevils who take the form of slightly devilish-looking humanoids. Fierna fights with a blade of fire she summons at will, and Belial battles with a powerful ranseur. Fierna is Belial's daughter; he lets her wear the mantle of leadership in public, but really the two rule the layer of Phlegethos together.
No one knows what Levistus looks like, because the prince is frozen deep within an iceberg. From the exterior, the prince looks like a great dark clot blearily realized through the ice. Asmodeus originally imprisoned Levistus in ice for a past betrayal. Though he allows the prince control over Phlegethos via mental contact with other devils, Asmodeus will not free Levistus. Despite his imprisonment, the plots of Levistus run deep. Should Levistus break free, his vengeance on the other lords of the Nine Hells, especially Asmodeus, would undoubtedly follow.
The Hag Countess is not an archdevil but a supremely powerful night hag from Hades. Moloch once ruled this layer, but was cast down when he defied Asmodeus during the general rebellion that ripped through the Nine Hells some time past. Most authorities acknowledge that the Hag, one of Moloch's most trusted advisors, counseled Moloch to rebel. Of course, the rebellion failed, and during the Reckoning the Hag was set in Moloch's place. Moloch fled, and wherever he is hiding, it is certain that this archdevil bears a terrible grudge. The Hag Countess's stronghold is the core of a mountainsized boulder that perpetually rolls down Malbolge's slopes, though none of her nobles know exactly where it is or on what path it rolls. Instead, they host her when she occasionally visits them in their copper fortresses, assuming they can pierce her expert disguises.
Once, Baalzebul was an archon of Celestia called Triel, but he was cast down into the Nine Hells. Asmodeus quickly promoted Triel to devilhood in a dread ritual. Baalzebul's relentless drive for perfection served him well as climbed the ranks of devilry, culminating in Baalzebul's ascension to the lord of the Seventh. And Baalzebul didn't just attain his lordship—he deposed the earlier lord and expunged all mention of that entity. Leaving behind his former name with his former allegiance, Baalzebul is also called the Lord of Flies because not even a fly can escape his net of intrigues. Like virtually every other lord, Baalzebul once plotted against Asmodeus during the failed rebellion and purge known as the Reckoning. To punish Baalzebul, Asmodeus transformed his formerly angelic body into a melted, sluglike form. Now, Baalzebul openly plots against only Mephistopheles, Lord of the Eighth. But his undying anger yet burns for Asmodeus, as does his undying ambition for the throne of the King of the Nine Hells.
Mephistopheles once engineered his own coup, when he was replaced by Baron Molikroth. However, Molikroth was secretly Mephistopheles, and that duplicity has now ended (as have the lives of “Molikroth's” co-conspirators. The Lord of the Eighth appears as a 9-foot-tall humanoid with hell-red skin, horns, wings. He grips a magic ranseur that burns eternally. He prefers to drape himself in dramatic capes as black as the deepest void. Like the other archdukes of the Nine Hells, Mephistopheles failed to unseat Asmodeus's rulership over the Nine Hells. And like many other archdukes, he retained his position when the Reckoning was over. His biggest rival is Baalzebul, and his court is thick with plots against the Lord of the Flies.
Asmodeus is at the very least an archdevil, but he could possess the power of a true deity. He resides in Malsheem but is rarely seen. Only when the elite of the Nine Hells gather in anyone of a hundred audience chambers does Asmodeus make his presence known, usually in the form of an unprepossessing humanoid with black hair, a black goatee, and eyes of flame. Asmodeus has been challenged many times, most recently during the Hells-wide rebellion called the Reckoning, but he has never been defeated.