Diablo ruled the hell for ages alongside his brothers, but was betrayed by some of them and imprisoned by a order of warrior known as the Horadrim. Diablo spent two centuries slowly corrupting the Soulstone that imprisoned him. In time, he was able to extend his influence into the surrounding area and possess both King Leoric and his archbishop, Lazarus.
The King proved too strong to fully possess, so the Demon took his son, Prince Albrecht. Diablo then began to shape an outpost of Hell within the catacombs that ran beneath the town of Tristram. By spreading terror into the surrounding countryside, the Demon was able to attract many heroes who came to cleanse the land of evil. By the time the strongest of these heroes reached this goal, though, he had become fully influenced by the power of Diablo. In his twisted state, this adventurer believed that the only way to fully control the Demon was to plunge Diablo's Soulstone into his own head. This, of course, was exactly what Diablo had planned as the Demon now had an even stronger body to use to find his brothers and complete his ultimate plan...
Diablo, after struggling with the adventurer's soul for some months, manage to take complete possession of his body, starting his quest to reawaken his brothers. He and his brother's banishment were actually all a set-up, as their plan were to be imprisoned all along in the soulstones, as they would be able to reach the Worldstone, a celestial artifact of enormous power, thus using the heaven's power against themselves. Diablo, disguised as the Dark Wanderer, traveled throughout Sanctuary and managed to release his brothers, Mephisto and Baal from their confinement. Once together the three brothers set a plan into motion to corrupt mankind. Diablo begins to spread terror but is stopped by a group of adventurers.
After 20 years it seems as though peace has finally come. All the great evils (excluding Azmodan) have been captured. All the spirits have been placed into the Black Soulstone. After Azmodan's failed attempt he too is trapped within. However this was all part of the plan. Adria the witch betrays the group and uses Diablo's daughter as his vessel. Temporarily using Leah's body he summons the army of hell. Arriving at the gate of Heaven, he is confronted by the Archangel of Valor Imperius. Using his spear Solarion, Imperius rips apart Diablo's "fake" form to reveal his true self. A monstrous reincarnation of Tathamet.
His appearance changes drastically as he has all the traits of all the prime evils.
Iconic spikes growing from the spine, red color of skin, as well as spiked tail, from Diablo.
Wing-like bladed spikes towering above the head and growing from the arms, from Baal.
One pair of horns and opened ribcage from Mephisto.
Breasts and hooves from Andariel.
Insectoid jaws from Duriel.
One more pair of wing-like bones and one more pair of eyes from Azmodan.
Two more mouths on shoulders from Belial.
Shape of the 'crown' horns that are growing directly from the skull appears to be a combination of horns that were once beared by Belial, Baal, Mephisto and Diablo.
After severly wounding Imperius and ripping apart the walls of heaven with his scream he begins his invasion. As he rips through the angels he opens portals directly from Hell into Heaven unleashing his minions. He captures Auriel and goes forward to annhiliate and corrupt the remaining portions of heaven. That done he attempts to corrupt the very essence of all angels in the crystal arch.
As the Nephaelem arrives to stop him he/she defeats all his lords. This causes him to add their power to his. By the time the Nephaelem reaches him he is all the evils together. With all his power his is still defeated and killed by the Nephaelem. As his body falls from the sky and disintegrates the black soulstone reappears.
Diablo III: Reaper of Souls
Although he doesn't appear here his spirit is consumed by the angel of death Malthael. When Malthael dies he is released once more.