Sargeras decided he needed his own army to fight the Void Lords, and he remembered the demons. He had defeated them and imprisoned them on a world he created called Mardum, the Plane of Banishment. Since demons can't be killed outside of the Twisting Nether, Sargeras created Mardum to store their souls, so that once the demons regenerated phyiscal forms, they'd already be stuck on Mardum.
He traveled to Mardum and shattered the planet, causing a massive explosion of fel energy which hideously disfigured him. Now transformed into a demonic creature of fel magic, he used his new power to bend the demons to his will. The destruction also tore a hole in the Great Dark Beyond, forming a gateway to the twisting nether. Now countless demons could stream out at all times, whereas before demons were a relative rarity. Sargeras part 2: Panetheon finds out.
After forming the Burning Legion, Sargeras went on to destroy a world for the first time. It didn't have a world soul, but it had been ordered and thus had a constellar like Algalon watching over it. When the constellar came to see what had happened, Sargeras killed him.
The Pantheon was confused that their constellar hadn't reported back, so they sent Agrammar to investigate. He saw the Legion destroying the planet, and Sargeras leading them. Outraged at his former masters' betrayal, Agrammar challenged Sargeras to single combat. They fought, but Sargeras was far more powerful, as titans are naturally weak to fel magic. Both of their blades split in two, and Agrammar fled to warn the Pantheon.
The Pantheon decided to hold a meeting with Sargeras over a world called Nihilam. They thought he could still be reasoned with. At the meeting Sargeras again insisted what he was doing was the only way to stop the Void Lords.
Agrammar sensed there was still a noble soul beneath Sargeras' madness, and walked up to him, unarmed, pleading for Sargeras to listen to reason. He said they had found a new world soul, of a titan named Azeroth, who would one day be strong enough to destroy the Void Lords. Sargeras used his sword to slice Agrammar in half vertically.The Pantheon was outraged over the murder of fellow titan, and fought a massive war with Sargeras.
His fel magic was too powerful and Sargeras killed them all with a massive storm of fel fire. Little did he know, Norgannon had cast a protective shround which perserved the remaining Pantheon members' spirits, so that as their mortal bodies were destroyed, their spirits would live on.
Nihilam came to be known as Doom World, the site of the Legion's destruction of the Pantheon. Even worse, Sargeras now knew the existence of Azeroth, of a world soul powerful enough to defeat the void lords. He didn't see it that way, he saw it as a world with the potential to birth the ultimate evil if corrupted by the Void Lords first. So he set out to destroy it.