The Void is both the name of an uberlevel complex accessed by using Dark Portal after killing Diablo on Destruction difficulty, as well as its final boss room, accessible via a one-way portal in the Plane of Pride, the eighth and penultimate area of the complex.
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.
Points of interest
The Void complex consists of nine areas in total: the entrance, called the Black Abyss, the seven Planes, each of which houses a boss, and the main boss room, simply called the Void. When defeated, each boss of the first six Planes drops a scroll, while the Soul of Pride, boss of the Plane of Pride, drops the Ring of Pride. The Ring of Pride and the six scrolls can be cubed to create the Sigil of the 7 Deadly Sins unique ring.
While the Void boss room starts out empty of monsters, its centre is the only place where the oskill Scream of Trag'Oul, granted by the ring, can be used, which will spawn the final bosses of the complex: Rathma, Disciple of Trag'Oul and Mendeln, Apprentice of Rathma, who will fight each other as well as the players.
The Demon Key, a guaranteed drop from Mendeln, grants one charge of the Into the Void oskill, which opens a portal to the Void if used in Act 4 Destruction difficulty, allowing players to skip half of the River of Flame, the Chaos Sanctuary and the entire Void complex if they want to re-attempt the boss fight against Rathma and Mendeln.