"With Aggramar able to hold his own, Sargeras had time to study the Twisting Nether's properties and find a way to contain demons. Though he hadn't yet grasped the volatile realm's full scale, he had learned how to manipulate and shape some of its energies. Sargeras used this knowledge to forge a prison within the Nether. Known as Mardum, the Plane of Banishment, it would act as a pocket dimension from which nothing could escape. No longer would demons be reborn after defeat. Now, they would be contained within this prison, where they would languish for all eternity. As Aggramar and Sargeras continued their campaign, the prison overflowed with demons and their fel energies. Soon, these energies became so great that they began tearing at the veil between the Nether and the physical universe. The prison appeared in distant reaches of the Great Dark as a burning, verdant star.[11]"
"Sargeras, having broken all ties with the Pantheon, meditated on the fate of the universe. He finally concluded that the only way to spare the universe was to purge it in fire. Thus his grand Burning Crusade would begin. To accomplish this Burning Crusade, Sargeras required a vast force of unquenchable rage. He knew of only one place that held such power and potential: Mardum, the Plane of Banishment. Over the ages, the prison became bloated with fel magic and vengeful demons. Their presence had warped Mardum, transforming it into a realm of nightmare. Sargeras quelled his remaining apprehension and tore the prison asunder. The subsequent explosion of fel magic was powerful beyond even what the titan had imagined. Violent energies enveloped Sargeras, surging through his veins and searing his very soul. His eyes burst in gouts of emerald fire. Fel volcanoes ignited across his once-noble form, splitting his skin apart and revealing a furnace of hate.[23]"