The Void is an endless empty realm where individuals can be trapped. The Void is described as being endless darkness, blacker than black and completely empty (like outer space but without stars or planets).
Lilith was twice banished into the Void by Inarius, though escaped the first time. Inarius also banished Rathma into the Void during the Sin War. Trag'Oul tried to retrieve him but he was too far out of Trag'Oul's reach, so he sent Mendeln in.