Angels have famously fallen, so why can't a demon rise?
At the beginning of Creation, when the High Heavens and the Burning Hells were still getting their bearings on existence, there was a demon that was filled with an uncontrollable, blinding rage. His body constantly burned the embers of Hell from the pure anger that filled him. Azmodan, the Lord of Sin, soon adopted the demon into his realm after seeing the personification of one of his many vices and made him a lieutenant of his, representing the aspect of Wrath.
However, the demon's fury was so great that he very quickly began to rebel against his own masters. He gave himself up to a small group of Seraphim in the Fields of Eternity, pledging his eternal loyalty to their kind for an opportunity to deny the Burning Hells even a small piece of victory. In their naievete, they accepted.
They took him to the High Heavens, and in secret, changed him. They shed as much of his demonic shell as their Angelic power allowed and created for him a shell similar to that of their Seraphim brethren. In fear of the great demon, they managed to quiet some of the rage, but they could not, however, extinguish the fire that burned everlasting. They trained him to fight like an Angel, move like an Angel, and act like an Angel. Soon enough, he was more than just a demon in angel's clothing; he became an entity of the High Heavens itself, bound to the Crystal Arch and allegiant to the Light.
Azmodan had assumed that his lackey had been slain by Angels, but had expected the Black Abyss to have spat him out again by now, but he was nowhere to be found. Azmodan was too preoccupied in controlling his armies against the Seraphim to pry, but Diablo took interest in the missing demon. He personally made a visit to Pandemonium and captured a few Angels to torture in Hell. As it just so happens, the Angels were some of those that rebirthed the demon of Wrath into one of their own kind.
It did not take long for The Lord of Terror to break the average Seraphim. The information he uncovered from the tortured Angels was much more than what he had originally expected. He was intrigued and frankly, impressed at not only the Angels', but the demon's boldness. Being the sly devil that he is (sorry for that pun), he decided that such valuable information would be better left off dwelling in his mind only, rather than those of his treacherous brothers'. Instead, he told Azmodan that his lieutenant had been captured by Heaven and imprisoned there for all of eternity.
The risen demon made short work of his former brethren in the battles on Pandemonium. Channeling the rage inside of him, he proved to be an unrelenting and impossibly absolute warrior of Light. He gained great recognition within the Seraphim ranks and beyond, but his legacy was just beginning.
He used his knowledge of his former master's strategies to outwit him at every junction. He eventually pushed the armies of Hell back all the way to the Burning Gates; the first time such progress was made by either side since the Dawn of Creation. Diablo was amused to watch Azmodan fail in battle, but was forced to push back when the war was brought to his doorstep. He did not, though, ever mention to Azmodan, nor anyone the secret about the warrior that had nearly breached the Burning Hells.
Even though the up-and-coming warrior fought harder and more valiantly than any angel before him, his battle at the gates of Hell eventually, and inevitably, failed. However, when he returned to the High Heavens, he was greeted with insurmountable respect. He was the High Heaven's answer to the Burning Hells. He was the unmatched hero of the Light.
Because of this truth, The Archangel of Valor was humbled by the champion and recognized that he was a better warrior than even himself. In the name of Justice, Wisdom, Hope, Fate, and most importantly, Valor, he proudly gave up his position on the Angiris Council and his status as an archangel to an angel much more deserving than he.
Little was known about the hero and new Archangel of Valor other than his exploits in battle and that every time it was mentioned in the Burning Hells, demons fled to find comfort in the realm of Anguish. Nevertheless, the Angiris Council welcomed the newcomer on board with open arms, looking forward to a future where their new champion, Imperius, The Archangel of Valor, vanquished evil forever.
Originally Posted by Diablo