The History of Azeroth
When Azeroth, the world of Warcraft, was young, the planet was tended to by the Titans, huge beings similar to Greek or Roman Gods. In fact they were actually referred to as The Pantheon. Their names were Aman’Thul, Eonar, Norgannon, Khaz’goroth, Aggramar, Sargeras, and Golganneth. Each had different powers and abilities that they used to shape the face of Azeroth. When they discovered the Old Gods corrupting Azeroth, fearing that their great size would hurt the planet-soul if they attempted to intervene directly, each Titan gave a portion of their power to the Titan-Forged, avatars they had created who lived on Azeroth. (and were much smaller)
Aman'Thul was the leader of the Pantheon, the oldest and the wisest and the one who was the first to “awaken” and search out the others. He had two avatars that he imbued with a portion of his power, Ra and Odyn. He channeled his power through Ra to give Nozdormu the power over time.
“Unto you is charged the great task of keeping the purity of time. Know that there is only one true timeline, though there are those who would have it otherwise. You must protect it. Without the truth of time as it is meant to unfold, more will be lost than you can possibly imagine. The fabric of reality will unravel. It is a heavy task--the base of all tasks of this world, for nothing can transpire without time.”
Eonar was Aman’Thul’s consort and the patron of all life. She gave a portion of her power to her avatar Freya, who was in charge of all the plants and animals of Azeroth. She also channeled some of her power through Freya to give Alexstrasza and Ysera their abilities.
To Ysera: “You are bound to the waking Dream of Creation. Nature is your realm, and all things have caught glimpses of the Emerald Dream when they sleep. You see them all, Ysera. And they see you, though they may not know it. Like the Life-Binder, you touch all living beings, and sing to them the songs of creation and interconnectedness.”
To Alexstrasza: “This is my gift: compassion for all living things. A drive to protect and nurture them. And the ability to heal that which others cannot, birth what others may not, and love even the unlovable--who surely need such grace more than any other souls.”
Norgannon was the keeper of Celestial magics and lore, he created the avatars Loken and Mimiron and channeled his power through Loken to give Malygos power over arcane magic.
“I believe that you will find that my gift to you is not just a profound duty—which it is—but also a delight—which it is! Magic must be regulated, managed, and controlled. But it must also be appreciated and valued and not hoarded. Such is the contradiction you must deal with. May you be dutiful ... and joyous both.”
Golganneth the Thunderer was the lord of the skies and roaring oceans, and Aman’Thul’s son. He is the god of the Sea Giants of Azeroth and he created the avatars Thorim and Hodir, giving them reign over the storms and skies.
Khaz’goroth is the Titan shaper and forger of worlds, he helped to shape Azeroth and designed the Un’Goro Crater to be an area for experimentation. He imbued his avatar Archaedas with his mastery over earth and forging, and channeled power through him to give the Dragon Neltharion his power over earth and the deep places of the world.
“My blessing upon you will seem humble compared to those which have been bestowed upon the others: the managing of time, of life, of dreams and magic. I offer you the earth. The soil, the ground, the deep places. But know that the earth is the basis of all things. It is where we are rooted. Where you must come from, if you are to go to. Here is whence true strength comes. From deep places...within the world, and within oneself.”
Sargeras was the Defender, the warrior of the Titans who was tasked with fighting the demons who escaped the Twisting nether and threatened the mortal realm and all the Titans had created. After fighting for eons, Sargeras realized that he was not defeating the demons, only returning them to the twisting nether, where they could eventually reform. Concerned about this, the Pantheon sent the young and inexperienced Aggramar, to be Sargeras’ lieutenant. Now that he had help in his endless battle, Sargeras was able to study the demons and eventually created the prison world, Mardum, to contain the demons he had defeated.
Eventually Aggramar and Sargeras split up to more efficiently battle demons across the universe. While they were apart, Sargeras discovered a world-soul that had been completely corrupted by the Old Gods. He had long suspected that a darker power lay behind the actions of the demons and he tortured those he found on this Dark world until they told him the truth. The Void Lords that resided in the Twisting Nether, had sent the Old Gods out into the universe to corrupt any world-souls they could find, with the intent of creating a power that the Titans themselves could not stand against. A Dark Titan.
Horrified, Sargeras immediately cleaved the world in two, killing the unborn world-soul, the baby titan.
He went to tell the Pantheon of what he had discovered and they were horrified at what he had done. They told him there must have been a way to cleanse the Titan, to save what they saw as their family. Sargeras was frustrated, they had not seen what he had seen, a Dark and powerful enemy that would have destroyed them all if it had awoken. He tried to convince them that they needed to destroy all life and start over. This was the only way to purge the Universe of the influence of the Old Gods and their masters, the Void Lords. Eventually, after much arguing, where Eonar fought for the preservation of life, he realized that they would never see things as he did and stormed off, never to return.
This is why Sargeras hates Azeroth so much. Azeroth is a world-soul. Inside slumbers a baby titan, but it is corrupted by the influence of the Old Gods. The Pantheon, when they discovered this, attempted to restore Azeroth and fight off the influence of the Old Gods, first physically with their own powers (pulling Y’shaarj out of the planet from space) and then when that went disastrously, through their avatars and the Dragon Aspects.
When the Old Gods were finally imprisoned, the Titans left to find Sargeras and confront him. For a long time in the lore we didn’t know what happened to them, or if they would ever return. They won’t. Turns out Sargeras killed them all. All that remains is the portions of their souls that they were able to send to their avatars, tied to them through the power they shared.
Sargeras, overcome with fear, doubt, and despair reached the conclusion that he must purge the universe of all life, the only way to be sure of the removal of the Void Lord’s influence. Thus began the Burning Crusade. Knowing he would need an army, Sargeras returned to Mardum and ripped open the prison, releasing so much fel energy he was horribly corrupted and changed. Bestowing the demons with power and a greater intelligence he gave them one choice, join him or be destroyed.
There was a powerful demon that refused his offer, he had been waiting for Sargeras’ return, growing in power and influence and plotting his revenge. Dreadlord Ulthalesh. In answer to the challenge, Sargeras forged a great felsteel scythe from the fires of Mardum's destruction, using it to harvest the souls of the rebellious demons and trapping them in a new prison. Ulthalesh, the strongest, was last to be devoured. Sargeras named the scythe after him. Ulthalesh, the Deadwind Harvester.
"The scythe's master will either overcome Sataiel or be consumed, which will only make the blade more powerful. And then another bearer will take their place. And another."
(The weapon is now an artifact wielded by Affliction Warlocks in Legion)
With his army, Sargeras invaded his first world, killing the constellar that the Titans had left to watch over the planet and erasing it of all life. This alerted Aggramar who came to see what was going on. After confronting Sargeras he quickly came to the conclusion that he would not be able to change his mind so he challenged him in single combat.
It was a terrible battle that no one won, but both were wounded grievously. Eventually Aggramar fled the battle to report to the Pantheon and get help.
The Pantheon and Sargeras met near the planet Nihilam. Aman’Thul pleaded with Sargeras, telling him of the world-soul Azeroth that was the most powerful Titan that they had ever discovered. It would be their saviour. When it awoke it would be so powerful it could challenge even the Void Lords.
Sargeras would not listen. Azeroth was an even greater threat to him, and he wanted it destroyed. Finally Aggramar approached him, unarmed, and tried to reach him through tales of all the battles they had fought together. In a fit of rage and fury Sargeras cleaved him in two. outraged by this murder the Pantheon attacked. They lost. Only the spell of Norgannon saved their souls, sending them to the avatars they had left on Azeroth.
Sargeras was now unopposed but he had lost much of his army in the fight against the Pantheon, and demons aren’t very smart. He started to look around for a strong and intelligent race that he could use to lead his armies and find more beings to conscript to his cause.
This is the point at which he discovers the Eredar and their leaders, Kil’jaeden, Archimonde and Prophet Velen. Kil’jaeden The Deceiver was tasked with finding new races they could corrupt and Archimonde The Defiler lead Sargeras’ great armies while Velen fled with the eredar that were not deceived or corrupted.
Who knows how many worlds Sargeras destroyed, but eventually he made his way to Azeroth, drawn by the power of the Well of Eternity, the titan’s lifeblood, when Queen Azshara started messing with it.
And we know how that ended.