Azmodan returned to Diablo in Diablo 4 Season 11 as a World Boss, the first since the game lauched in 2023. He also played a part in the Season storyline in Season 11 taking over the Woeld Boss arena in Hawezar.
Azmodan, the Lord of Sin, is one of the seven Prime Evils and one of the four Lesser Evils, along with Belial, Andariel, and Duriel. During the Eternal Conflict Azmodan won many battles against the High Heavens, and gained renown as the most capable battlefield commander of all the Lords of Hell; his ability to surpass Baal was only due to the latter's volatile nature and brashness in war.
As the Prime Evils turned their attention to the mortal world of Sanctuary, the Lesser Evils grew restless. Eventually, Belial, the Lord of Lies, manipulated Azmodan into leading a revolt against the Prime Evils. Azmodan is best known for events during the Sin Wars, when his alliance with Belial gave their combined demonic armies enough strength to defeat and banish the three Prime Evils into the mortal realm. (There is some speculation that the Prime Evils engineered their own exile and subsequent capture by the Soul Stones as an elaborate plot to ultimately gain more power.)
Azmodan and Belial's triumph was brief, for they fell back into war with each other as soon as they no longer had to cooperate against the Prime Evils. Azmodan's tactical acumen and overwhelming force allowed him to dominate the Civil War and defeat his rivals, including Belial.
From the early information we have about the plot of Diablo III, it seems that Azmodan has become ascendant in the Burning Hells, and is acting as the leader of the realm in leading a massive demonic army to invade Sanctuary; one of the key plot events of Diablo III.
The Lord of Sin
Azmodan as a physical being is an incarnate of all seven of the deadly sins: wrath, greed, sloth, price, lust, envy, and gluttony. He also controls powerful demons who personify each of those sins, two of were seen in the initial version of Diablo 3.
Belial and Azmodan in the Diablo I Manual
Azmodan is featured alongside Belial in the Diablo I Manual when the civil war between the lesser and prime evils is explained:
In their ignorance, the Lesser Evils began to believe that the Three were afraid to continue the war with Heaven. Frustrated by the cessation of the war, Azmodan and Belial saw the situation as their chance to overthrow the Prime Evils and take control of Hell for themselves. The two demon lords made a pact with their minor brethren, assuring them that the wretched plague of humanity would not deter the ultimate victory of the sons of Hell. Azmodan and Belial devised a plan to end the stalemate, achieve victory in the Sin War and ultimately ride the bloody crest of the Great Conflict straight into the very arms of Armageddon. Thus, a great revolution was set into motion as all of Hell went to war against the Three Brothers...
The Brothers fought with all of the savagery of the Underworld, and to their credit, annihilated a third of Hell’s treacherous legions. In the end, however, they were overcome by the Horned Death led by the traitors Azmodan and Belial. The Prime Evils, weakened and bodiless, were banished to the mortal realm where Azmodan hoped that they would remain trapped forever. Azmodan believed that with the Three set loose upon humanity, the Angels would be forced to turn their focus upon on the mortal plane - thus leaving the Gates of Heaven abandoned and defenseless. Those few demons who still pledged allegiance to the Three Brothers fled the wrath of Azmodan and Belial, escaping to the realm of Man to seek out their lost Masters.
As the warfires died out upon the battlefields of Hell, Azmodan and Belial began to argue over which of them held the higher authority. The pact that they had made quickly fell to ashes as the two demon lords took up arms against each other. The legions of Hell that remained were polarized behind either warlord, launching themselves into a bloody civil war that has lasted to this day...