Origins
Old Gods
In the time before Humans, before High Elves, before the Night Elves and the dragons, before Sanctuary, there was nothing. When Sanctuary was formed by those who fled other worlds, two other worlds were born with it. In Heaven, creatures of purity and light gathered, becoming known as Angels. They soon found a lord in Tyrael, who led them kindly.
The other world, however, remained untouched by all but the darkest of creatures. One of these, the Lady of Darkness, often left her dark realm in search of a lover. The Mortal Realm had none that could woo her, and whenever she tried despite this, they would be enveloped by her darkness, never to return.
She searched the depths of Hell, her own realm, but none of those creatures wanted her love, but rather her dark energy. Desperate, she begged all living to find someone to become her lover. To her shock, Tyrael came down to slay her. The light of Tyrael began to cut the Lady of Darkness, as he shouted at her that any beast she was to spawn would destroy the world.
After that encounter, which ended with both parties mostly pleased, the Lady of Darkness had found a seed of light within her. It grew within her, until it had split into three. Each was a growing monstrosity mixed between Light and Darkness. She went to the Mortal Plane when it came time to birth them, but to her shock, an Angel came down when the children were born, and took from them their Light, saying that Light and Darkness could never mix. The Angel escaped the world he had damned through the blade of Tyrael.
The children of Darkness retained their power, for the angel had only taken their light, and therefore their peace. All that remained was the Darkness, which filled them. As she birthed them, she gave them names of ancient heritage.
First was Mephisto, a name meaning Hatred. The winds howled when he cried. The second was Diablo, referring to Terror and Evil simultaneously, as his makeshift crib burst into flames. Finally, Baal was born, shattering his earthy surroundings. Each would one day command these elements, along with the Lord of Fury, C'Thun, their lesser brother born of a separate mother.
The name of one evil has been lost, forgotten to all, but the other four are remembered by few. Their lieutenants, however, are a different story.
The destruction of the Worldstone revived each of the evils as mere spectres, capable only of possession, and all took special bodies.
Diablo chose the Wanderer, whose body had been preserved in hell, and restored. Manipulating the fires of hell, Diablo created Ragnaros the Firelord. The Terror of Fire spread across the living, and destroyed their homes.
Mephisto brought his Hatred across on the Winds through the shaping of Al'Akir the Windlord. The Winds of Hatred spread amongst men, bringing them to slaying one another, weakening his foes without facing them. Like his servants, he took upon the Wind as his body.
Baal, the Lord of Destruction, took the Earth itself to balance out his power. Creating Therazane the Stonemother, she gave birth to stone which destroyed life, and so the Destructive Stone crushed mortals as the Adventurer who had 'defeated' Baal watched on, the Prime Evil controlling him.
C'Thun alone retained his original body, as he was nowhere near the level of Prime Evil, though he did lurk in the depths, and was powerful. It was within the trenches of the deep oceans that he birthed Neptulon the Tidehunter, who spited the power held by the Prime Evils who created Ragnaros, Therazane, and Al'Akir.
Life could not stand before the power of the Elementals. Soon, even the heavens of Sanctuary were destroyed, and Hell collapsed upon itself to provide balance. All that remained of the taint was banished, separated forever from Sanctuary. Only the Prime Evils, who now called themselves Gods, remained.
AN: This is possibly my worst yet. Hopefully, though, it makes sense overall, and doesn't butcher the known history too much