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 during the events of the Sin War. Trag'Oul tried to retrieve him but he was out of Trag'Oul's reach, so he sent Mendeln in his stead. By Trag'Oul's own account, Mendeln was in grave danger as long as he was there, as he risked entrapment as well just by being there.
Inarius has also trapped Lilith there 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. Eventually she was once again sent there by Inarius.
Note that the Void is not the same as the Black Abyss. While the Void has been clearly established in the lore and is more like a place where beings can be trapped (like Lilith), the Abyss is an obscure place only mentioned twice in Diablo II. Little is known of it, but it is believed to be a place where demons are spawned from and where they return when they die.
Trag'Oul's realm is similar to the void in some aspects but is not the same place.
"Where--where is this?" he finally managed to ask. "And did you say that Rathma is here somewhere?"
Here... and trapped possibly until the end of all. Sent to this accursed place by Inarius as a reward for seeking to do the right thing.
Mendeln had feared that the Ancient's visit to his father would prove to be a fool's errand, but even he could not imagine the angel so vicious as to condemn his offspring to this hellish abyss.
- The Veiled Prophet, page 235. Richard A. Knaak, 2007.
|Locations in the Diablo Universe|
|Planes||Black Abyss • Burning Hells • High Heavens • Sanctuary • Trag'Oul's Realm • The Void|
|Regions in Sanctuary||Aranoch • Dreadlands • Entsteig • Kehjistan • Khanduras • Mount Arreat • Scosglen • Skovos Isles • Torajan Jungles • Westmarch • Xiansai|
|Towns in Sanctuary||Bastion's Keep • Bramwell • Caldeum • Duncraig • Gea Kul • Harrogath • Hashir • Istani • Ivgorod • Kalinash • Kehjan • Kingsport • Kurast • Lut Gholein • New Tristram (Old Tristram) • Partha • Rogue Encampment • Seram • Staalbreak • Toraja • Tran Athulua • Tulisam • Tur Dulra • Ureh • Urjhani • Viz-jun • Westmarch|
|Other Places||Pandemonium Fortress • Silver City|