"The Faithless Destroyer, The Hateful One, The Terrible and Irresistible, he is the fallen creator, like a fire that exists solely to destroy. His sight is far, his armies innumerable, and his rage scorches worlds to dust."
Sargeras, also called The Destroyer and The Brother of Hate, is the ancient being from long before the dawn of time who, alongside Anui, was known as a Brother of Order. He and his brother forged many of the realms of Creation as we know them, until he fell to the corruption of Y'shaarj, the black worm. He is the greatest foe of Anui, The Divines, and is an enemy to any form of order or goodness in all Creation. Sargeras is worshipped and feared all across the multiverses, and is the father of all evil, and all demons. He looks at all life with disdain, and he views creation as little more than a disease which must be purged.
Sargeras is the founder and undisputed lord of the demonic horde known as the Burning Legion. He uses this unstoppable force to do his personal bidding, including the conquest and destruction planets. Whether Sargeras does this out of duty or enjoyment is unsure. He is the father of the evil dragon Drakuljonak.
Sargeras' story began with the creation of Order - he was born alongside Anui at the dawn of Time, after Ehyeh's struggle against Y'Shaarj. The Brothers of Order were tasked by Ehyeh with being the guardians and masters of Creation. To do this, they created the stars, galaxies, and worlds, protecting them from the vile forces of Chaos. The Brothers combatted Chaos for eons, and after the slim victory against the lawless power, Anui and Sargeras created the Old Gods (Su'ada) to uphold the Order.
Sargeras created an immense army of terrifying demonic creatures, totally loyal to the mighty and ancient being. They were named the Burning Legion. He also employed an ancient and powerful leader of a mighty race of space conquerors. This leader was to serve as second-in-command of the demonic Legion. His name was Kil'jaeden.
With this mighty army at his back, Sargeras gained the upper hand in the war against the High Gods. This continued for many millenia, the mighty High Gods losing more and more ground to the vile forces of Sargeras. Eventually, the perpetual failure and years of war began to take its toll on Chakravartin; he began losing his sanity. He decreed that He and a few High Gods would travel outside of Araboth to take over a new Existence so that they could have a greater foothold in the war against Sargeras. This was around the time of the War of Deceit in Mundus. Sargeras was pleased with this; the absence of Chakravartin in Araboth would certainly weaken the High God war effort. Eventually, Chakravartin died in Mundus, which completely eradicated any hopes of the High Gods winning the war. Leaderless and demoralized, they either fled or were cut down swiftly by Sargeras' followers and the Burning Legion. Those that fled are still hiding somewhere in Araboth, or were hunted down in later years. The hunt for those that ran away continues today, as the responsibility of the Burning Legion. Due to the failure of the High Gods in the war, there is virtually no such thing as good in Araboth anymore. Good is a concept materialized from within some mortals, but it is not a widespread concept, nor is it upheld by any supreme beings. Evil and destruction are widespread here.
Sargeras, victorious, established himself as the lord of Araboth. Auriel, embodying good, justice, and peace, is a prime target for the warmongering god. However, he is waiting for the perfect time to strike against Mundus.
Since then, Sargeras has built up his demonic Burning Legion to invade Mundus, kill the gods, and destroy what remains.