The Void is an uberlevel complex, accessible through using the Dark Portal skill after Diablo is defeated on Destruction difficulty. It consists of nine sub-zones in total: the entrance, called the Black Abyss; the seven Planes of Gluttony, Lust, Greed, Sloth, Pride, Envy and Wrath; and the boss area, simply known as the Void.
With the creation of the Sanctuary by the angel Inarius and demoness Lilith, the demons and angels who wanted to avoid the Great Conflict between the High Heavens and Pits of the Burning Hells came with them and settled it. The first humans were created as their offspring by mixing the demonic and angelic – the Nephalem. At first they seemed to be nothing like their parent, but quickly it became evident that they wield latent magical powers possibly even greater than their ancestors. Finding this out, Inarius decided to kill them all, but Lilith wanted to make an army to fight the Heaven and Hell. They began fighting, and Lilith, defeated, was banished into the Void – an utterly empty and black space, a prison for various beings – to be eternally trapped there.
Rathma, their son, had avoided the slaughter that followed, and became the servant and student of a mystical dragon-guardian of the Sanctuary – Trag'Oul. After a thousand years, Lilith escaped the Void in the middle of the Sin War events, faming Uldyssian and making him fight the Triune and various related demons with his companions - one of them being Mendeln, his brother. Rathma took in Mendeln, and trained him as his apprentice – the two of them being the first Necromancers, following the teachings of Trag'Oul. Eventually, Rathma was found and sent to the Void by Inarius, but Trag'Oul sent Mendeln to retrieve Rathma at a great risk.