Rise of demons

The rise of demons is the conflict that lasted many millennia between demons of the Twisting Nether and Sargeras of the titan Pantheon, who was later joined in the conflict by Aggramar.

The conflict emerged after the rise of demons in the Twisting Nether and their terrorizing of mortal civilizations bringing ruin to world after world, and after the Pantheon learned of the demonic incursions that flared at the far corners of creation.

It's one of the earliest conflicts in the timeline of the Warcraft universe.

Prelude
The benevolent Pantheon, which sought to safeguard these structured worlds, was ever vigilant against the threat of attack from the vile extra-dimensional entities of the Twisting Nether. The Nether, an ethereal dimension of chaotic magics that connected the myriad worlds of the universe, was home to an infinite number of malefic, demonic beings who sought only to destroy life and devour the energies of the living universe. Unable to conceive of evil or wickedness in any form, the titans struggled to find a way to end the demons' constant threat.

The conflicts
When demonic entities made their way into the titans' worlds from the Twisting Nether, the Pantheon elected its greatest warrior, Sargeras, to act as its first line of defense. A noble giant of molten bronze, Sargeras carried out his duties for countless millennia, seeking out and destroying these demons wherever he could find them.

Sargeras continued defeating the disorganized and incompetent demons with ease, but he also soon discovered that some demons had begun wielding the dark energies of the Void. Through this discovery, the Defender also discovered the existence of the void lords, malevolent beings far more powerful than mere demons who sought to influence the physical universe. The presence of these void lords troubled Sargeras, but he nonetheless continued his work.

To his dismay, Sargeras soon discovered that the demons he had defeated simply returned to the Twisting Nether to be reborn in new bodies and continue to torment the worlds of the Great Dark. Sargeras was greatly troubled by the creatures' corruption and all-consuming evil. Incapable of fathoming such depravity, the great titan began to slip into a brooding depression. The Pantheon grew concerned about the increase of demonic activity, and so sent the inexperienced titan Aggramar to aid Sargeras in battle. Aggramar proved a quick study and soon earned Sargeras' admiration, becoming the warrior's trusted lieutenant.

The two fought shoulder to shoulder against the endless demons of the Nether. This allowed Sargeras to more closely study the nature of the Nether, and by learning how to manipulate the chaotic realm's magical energies, despite his moral growing unease, he created a prison world on a corner of the Twisting Nether to contain the warlocks and demons he had defeated. Demonic invasions became increasingly rare, and together, the two titans soon brought peace to the cosmos.

Sargeras and Aggramar eventually agreed to go their own separate ways in order to allow them to more effectively combat the demons. Eventually, over the eons, Sargeras encountered the nathrezim, a powerful demonic race which was bent on gaining power and dominance over the physical universe. While his confusion and misery deepened, Sargeras was forced to contend with them, whose intent was disrupting the titans' order. This dark race of vampiric demons (also known as dreadlords) conquered a number of populated worlds by possessing their inhabitants and turning them to the shadow. The nefarious, scheming dreadlords turned whole nations against one another by manipulating them into unthinking hatred and mistrust. Sargeras defeated the nathrezim easily, but their corruption affected him deeply.

Eventually Sargeras discovered the full extent of the void lords' plans. Coming across a world that had been almost completely infected by monstrous aberrations known as Old Gods, the titan ruthlessly interrogated a conclave of nathrezim demons who dwelled on the black world. From them, Sargeras learned that the void lords had sent the Old Gods out into the cosmos to infect any worlds that housed titan world-souls, and if they succeeded, the world-soul would be twisted into an unspeakably dark creature that not even the Pantheon could stand against. Enraged at this discovery, Sargeras cleaved the planet in two, instantly killing the nascent titan it housed. He then immediately returned to the rest of the Pantheon to tell of his discovery, but they chastised him for his rash action, insisting that they might have been able to purge the world-soul of its corruption without killing it. Sargeras tried to convince the other titans that what he had done was right, and that the universe needed to be purged of life in order to prevent the risk of a world-soul becoming corrupted and opening the way for the void lords. Realizing that the others would never see things the way he did, Sargeras stormed off, vowing to cleanse the universe on his own.

As doubt and despair overwhelmed Sargeras' senses, he lost all faith not only in his mission but also in the titans' vision of an ordered universe. Eventually, he came to believe that the concept of order itself was folly and that chaos and depravity were the only absolutes within the dark, lonely universe.