Asmodeus

Asmodeus, the King of Hell. Note the firebred hellhound and, yes, the Ruby Rod.

Asmodeus
Alignment	Lawful Evil
Divine Rank	Debatable (see below)
Pantheon	Openly Archdevils; Ancient Brethren secretly
Portfolio	Being Satan
Domains	Knowledge, Torment, Tyranny (4e), Evil, Fire, Law, Magic, Trickery (Pathfinder)
Home Plane	The Nine Hells of Baator (currently imprisoned), the primordial soup of creation (former)
Worshippers	Slavers, tyrants, bureaucrats, Satanists
Favoured Weapon	Mace (shaped after his Ruby Rod)

Asmodeus is THE evil god AND an Archdevil in Dungeons & Dragons. He is the absolute ruler of the Nine Hells, stationed squarely in its infernal core of Nessus. His daughter Glasya is one of the commanders in charge of a "lower" level.

Fluff

Asmodeus was introduced in AD&D 1st edition. In the Monster Manual, no less! His domain was detailed in Dragon magazine. It's mostly stayed the same since.

Asmodeus was part of the Creation War. After defeating the Primordials, the evil gods set out to kill each other over who would take over the Universe while the good and neutral ones tried to stop them. Asmodeus was able to get a large number of angels to side with him and led a rebellion against the heavens. He was struck down and landed on Baator, which originally had seven Hells but he made two more just for shits and giggles. There he set up his kingdom of hate.

Asmodeus takes on the form of a boring, run-of-the-mill Halloween devil. Oh, except for being 13 feet tall. He has impossibly high charisma and can only really be damaged with Holy items. Typically, he's a patient schemer who can wait epochs for his plans to resolve, but during the rare times he doesn't use stars forming and dying to time himself, he uses guile rather than force... usually. His Ruby Rod has a lot of at-will spells that he can use, and he can alter lesser beings. ...Yes. It's called the Ruby Rod. The Ruby Rod of Asmodeus. It's been around since forever, and, yes, it looks exactly how you think it looks. Some of his followers even carry similar Ruby Rods, but they are far inferior in length, girth, and potency to his.

This has been an obvious dick joke. Please, continue with the article. The best part is, that dick joke is actually official canon.

2e

The book Guide to Hell gave us a completely new view of Asmodeus. At the dawn of creation, beings of Law began to rise up from the primordial soup. The mightiest of these were Ahriman and Jazirian. Jazirian was a feathered and winged serpent, while Ahriman was scaled and forked of tongue. They bit down onto each other's tails and together began to create Law in the chaos. The first thing that they created was the Unity of Rings: everything keeps happening over and over again. This place, the first ring, became known as the Outlands. When it was defined the other planes fell in around it, thus creating the Outer Planes.

Next up after this was to honor their three aspects: Law, Good and Evil. From this came the Rule of Three, stressing the importance of the number three in all things. Finally there needed to be a third rule: the Center of the Universe. This is where it started to go wrong: while the Outlands would have been the obvious choice, Jazirian chose for heaven, while Ahriman chose for hell. The two began to pull and struggle at and with one another, until they eventually tore each other apart. Jazirian flew up to the heavens: each drop of blood she spilled from her severed tail tip turned into a fully-formed couatl. The wingless Ahriman though began to fall: he in fact fell so hard that upon hitting the seventh layer of hell, he fell through and in his crash created two new layers. Where his blood hit the ground, Pit Fiends sprung up. Ahriman got stuck in his pit, unable to leave.

Jazirian and Ahriman, busy creating the three rules of the multiverse.

In their struggle the two serpents had lost much of their power, and began to nurse themselves back to health. In this time, the other gods overtook them, banded together and began to colonize the planes. So, Ahriman began to hatch a new plot. He took the guise of Asmodeus, a near-power just short of being a god: the arch-devil and the lord of evil. But he is more than that: more potent than even the gods and not dependent on the adoration of mortals. He is trying to get into power by ways of three plans: the Blood War, the Politics of Nobility and Armageddon.

Asmodeus fakes that the Blood War is of great importance to him: he sees it as little more than a series of skirmishes who turned into a full-blown war. But by staying at war with the Tanar'ri he makes the upper powers think that if one of them were to win, they would flood out to the rest of the planes and create a massive war that would end all life. Thus, they continue to let the fiends fight amongst themselves. The second plan, the Politics of Nobility is more or less the same thing but internal: by keeping the devils occupied with struggling with one another they won't have the resources to fight Asmodeus and his plots.

The third one is a far more sinister plot. Those who die without believing in anything. Since only the believers can go to an afterlife, those who do not believe are instead send elsewhere, namely Nessus. But instead of becoming petitioners here and starting out as Lemures, they become chow for Asmodeus. The process of being eaten by a primordial serpent takes centuries, during which the petitioners remain fully aware as they are devoured, feeling nothing but excruciating pain for centuries. Being destroyed in such a way completley undoes the soul in question and cannot be brought back in any way. Every soul devoured this way heals Ahriman's wounds a bit more, and when he has fully healed Ahriman will break free of his prison and cast the multiverse back into the primordial chaos it came from. Nobody, not even Jazirian would be able to stop him, and he would remake the universe to his liking. And this is a Bad Thing.

3.5e

In 3.5 Asmodeus got a new backstory and became a magnificent bastard of massive proportions. In the ancient days the Gods were troubled by mortals because they would do bad things over and over again. Asmodeus, one of the smartest people ever, pointed out it was because there was no consequence for doing this and invented "Punishment" which took the form of a rod with which to strike the guilty. The Gods liked the idea and so Asmodeus and his followers went about punishing mortals that did wrong all over the heavenly realms. Well the Gods of Good that they served got disturbed by this; sure there were results and mortals were being less of a collection of dicks because of it, but they didn't like evil souls being tortured in their herb gardens. Asmodeus had a solution. He wrote up a giant contract that would make him and his followers separate from the Gods, give him his own realm (a little one-layerer called Baator), and he and his followers would get energy from those they punish, leaving the Gods free from having to manage the thing. It was such a good idea that all the Gods signed it. When Asmodeus and his followers arrived at the dirt hovel plane after imprisoning its former ruler Zargon, they thought he had gone mad. Then he explained the rest of his plan.

A few millenia later, the Gods started to notice something was up with the number of souls they were getting. Investigating, they found that Asmodeus had been a productive bastard. 9 layers of hellscape, legions of devils, and innumerable tortured evil souls. Except some of those souls had been bought or corrupted by Asmodeus and his followers.

St. Cuthbert the God of Retribution drew forth his mace and shouted "You fiend! You are only supposed to punish the wicked, not tempt them into acts of evil!" Asmodeus smiled, held out the contract alongside a novelty oversized magnifying glass, and said "Read the fine print."

In this edition we see why Asmodeus re-installed Levistus to his former position as leader of the 5th layer of hell without freeing from his iceberg prison and more importantly, not providing the traditional promotion makeover.

4e

In 4th edition Asmodeus isn't the most powerful devil ever. He is the GOD who is in charge of the devils. That's right, the bastard managed to plot and assassinate his way to divinity. The only thing that can possibly stop him is if any mortal manages to, without divine aid, discover the name of the God he assassinated and stole divinity from and get enough worshipers of that God going to bring them back to life. Asmodeus killed every one of that God's followers and destroyed every mention of that God in mortal texts...and is constantly double checking to make sure he didn't miss anything.

In this edition, he's also the creator of all tieflings, as the human empire of Bael Turath turned to diabolism in order to get the strength to defeat the dragon-led dragonborn empire of Arkhosia. Though both empires eventually destroyed each other, by that time, the infernal pacts, diabolic interbreeding and sheer corruption had converted most/all of the Bael Turathi and their descendants into tieflings. To this day, tieflings are stereotyped as Asmodeans by default.

Pathfinder

Like in 4ed Asmodeus is a God, though rather than a Devil who worked his way up, he was either a God from the start or one of the oldest ascended Outsiders, and is one of the biggest players among the Gods. The Book of the Damned claims him to be one of a pair of primordial first gods with his brother Ihys, until the two had a falling out on the matter of law vs chaos, gathered allies among younger beings, and in the first act of deific treachery, Ihys was slain by Asmodeus. He then gathered his celestial followers and marched them to Hell, a far more empty place back then, and took it over, sculpting one of his Archdevils, Mephistopheles, out of the plane itself in his image. The 'primordial first gods' story could be BS, other accounts paint him as an ascended Celestial, but he's old enough regardless that only the later conflict and Hell parts can be confirmed by the other Gods.

As the god of tyranny, hierarchy and order, he places law as paramount and is surprisingly easy to deal with for an evil, at least if you yourself are a god. Even the younger and more hot-head good gods have consulted him on occasion. Most famously, when Rovagug the Rough Beast went to war with all the other gods, the Empyreal Lord Sarenrae forced it into a prison in the center of Golarion and Asmodeus provided the key, locking it away hopefully forever, but making his position to the Gods indispensable.

Asmodeus believes the entire multiverse will move past this whole 'free will' thing and fall under his tyranny eventually, but as such an ancient being he is willing to be supremely patient about it, make deals with everyone, and wait for the inevitable.

Followers

Yeah, anyone lawful evil basically. There are a few who worship other beings, but most of them go to this guy. He's very convincing. They're all gonna go to Hell. And while he might promise you something nice once you get there, read the fine print. It ain't worth it.