The 'Void' is a plane of existence where powerful entities or beings can be sent to be trapped for eternity, the void is described as being completely empty, containing no stars, planets or object in anyform other than utter darkness.
The Void is a mysterious realm where beings or entities can be sent by powerful forces to be trapped for eternity. The void is described as being endless darkness, blacker than black and completely empty (like outer space but without stars or planets). It is a horrible fate to be condemned to.
Rathma was sent there by Inarius, and Trag'Oul tried to retrieve him but he was too far out of Trag'Oul's reach, so he sent Mendeln in.
Lilith was trapped there by Inarius as punishment for trying to take Sanctuary for herself near the beginning of Sanctuary's creation. She escaped thousands of years later back to Sanctuary where she found Uldyssian.
Note that the Void is separate to the Black Abyss. The Void is more like a place where beings can be trapped in (like Lilith), while the Abyss is where demons are actually spawned from and where they return to when they die.
'Trag'Oul's realm is similar to the void but is not the same place.
Inarius raised one palm. In it formed a gleaming sphere so transparent that it was almost invisible. The demoness’s expression turned to horror. “No! Inarius! Don’t—” But in the next second, Lilith floated inside the tiny sphere, her size reduced accordingly. I HAVE REMEDIED WHAT WAS DONE INADEQUATELY BEFORE, the winged being said without emotion. THE MISTAKE SHALL NOT BE REPEATED. GOODBYE, MY ONCE LOVE. She spat at him although the sphere prevented it from having any result. “You think Sanctuary yours? You see what this human has wrought! He’ll bring you down, too, Inarius!” HE WILL NOT, FOR IT IS THROUGH MY EFFORTS THAT HE HAS DONE YOU IN. Before she could argue more, Inarius added, FAREWELL, MY ONCE LOVE…FAREWELL… Lilith screamed and cursed, but her voice—as well as she—grew tinier and tinier. The sphere became a marble, then the size of a pea. And then, for all mortal purposes, shrank so tiny as to become nothing.'
references:
Diablo - The Sin War - Book 02 Scales of the serpent Richard A Knaak 2007 [1]