The realm we know today as The Nine Hells is also known as Baator. Before the devils arrived, creatures known as the baatorians dominated the plane They had a hierarchy much like modern devils do.
In the earliest days of known history, the gods began dividing up and populating the planes. This created a number of conflicts. Corellon and Gruumsh did battle. From Corellon’s tears came the elves, and from Gruumsh’s blood came the orcs. The deity who looked over humans was known as He Who Was, or He Who Lit Our Way.
It is said that He Who Was created humans in his own image. This deity had a host of good and just angelic servants who were tasked with watching over humanity.
His angelic servants included Asmodeus, Malcanthet, Mephistopheles, Geryon and many more entities who today are known as some of the biggest villains in the multiverse.
Asmodeus was the most powerful warrior in service to his god. Over time, the efficiency of Asmodeus gave way to brutality. Asmodeus got involved in a human conflict and participated in a massacre in which innocents had fallen alongside enemies.
Asmodeus and his cohorts were demoted as punishment. Filled with remorse, Asmodeus rededicated himself to being an angel of protection, virtue and chastity. His new duty was to guard the prison of the mad god, Tharizdun. Pazuzu, an ancient obyrith (obyriths were an eldritch race of demons so ancient that they predated mortal life, and even the existence of the gods, simply looking upon their strange shapes could drive a mortal insane, and even the cosmos itself was horrified by their presence) and demon lord, began to quietly visit him. He went about sowing the seeds of corruption in the angel, telling him that He Who Was was mis-using his talents and that he was above serving as a lowly guard.
Asmodeus already was having misgivings about his deity. He Who Was had many unhappy spouses, his treatment of humans was often harsh and callous, and he selfishly created certain types of angels specifically to serve him.
Asmodeus began hearing whispers from the seed of evil buried in the heart of the Abyss, planted there by obyriths such as Pazuzu long ago. Asmodeus abandoned his post, broke a small shard off of the seed and placed it in his scepter.
The shard turned the soul of Asmodeus black. Fully corrupted, Asmodeus began recruiting angels to enact a rebellion against He Who Was. It was not overly difficult for him to rally disgruntled celestial knights and angels of protection. Many of his fellow angels were swayed by the fervor of Asmodeus and joined him.
The Succubus Queens, Malcanthet and Lilith were two of the most powerful angels to join his cause. Asmodeus was able to lure them to join the rebels by offering them his pure, chaste, angelic love. He made both of them phony promises and played them off of each other.
It is believed that the angels of He Who Was won the conflict that came to be known as the Battle of Blood-Dimmed Stars. At the end of the bloody battle in the night sky, an angel loyalist of He Who Was named Alloces captured one of Asmodeus’s officers. Alloces so relished torturing the rebel that his fellow angels were repulsed and rebuked him. Alloces realized that he had had more in common with Asmodeus and decided to defect.
It is said that Asmodeus won the Battle for the Bridge of Storms. What turned the tide for him was that Geryon betrayed He Who Was and joined the rebels. Amon, who had been a staunch loyalist of He Who Was, swore an oath of fealty to Geryon right there on the battlefield. Thousands of years later, Amon still honors that oath.
A key turning point in the rebellion was when Asmodeus convinced Bensozia, one of the wives of He Who Was, to betray her husband. He secretly promised her his chaste love, as he had also done with Malcanthet and Lilith.
An unheralded key ally in the rebellion was Beleth, lord of imps. Beleth was a traitor among the angels of He Who Was, secretly feeding Asmodeus information culled from his network of imp messengers. Eventually Beleth abandoned He Who Was and joined Asmodeus.
Most prominent angels known to have joined Asmodeus in the rebellion:
In a final, world-shattering battle, Asmodeus mortally wounded He Who Was. The god cursed Asmodeus with his dying breath, imprisoning him and the traitors in Baator (The Nine Hells) forever.
Asmodeus fell through the planes to Baator, exploding through eight layers. His titanic, angelic form shattered as it struck the bottom of one of the deepest rifts of Nessus. From that shell emerged Asmodeus in his new infernal form.
The Angelic Husk. Some believe that the shell is his true form, slowly healing. The creature known as Asmodeus might actually be an avatar of an even more powerful entity.
During the battle with He Who Was, Asmodeus obtained bloody wounds that have never healed. It is said that a pit fiend is born from each drop of his blood.
Many of the other angels were banished to Hell as well. They plunged into the lake of fire in Phlegethos.
Baator (The Nine Hells) was not a barren realm. It was home the evil baatorians, who at the time were led by an incredibly powerful beast known to some as Zargon. Still armed with the ruby rod containing the shard of the seed of evil, Asmodeus went to war with them.
Asmodeus killed some of the most powerful baatorians and drove away others. Some went into hiding deep underground, some fled to the planes, and some hid in plain sight.
With Hell conquered, the fallen angels pulled themselves free of the lake of fire, and settled into their eternal prison, remorsefully. Many of the fallen angels began to regret their what they’ve done, but nothing they could do to repent, though some began to scheme against Asmodeus, who caused their woes in the first place.
Using the power of the divine spark he had drawn from the god in Phlegethos, Asmodeus gave his fallen angles the power to transform the strange nupperibos (humanoid with liquid flesh hanging from the bone) into creatures more to their liking. He assigned his fallen angels’ layers and designed a hierarchy that ensured they were at odd with each other rather than him.