The Void Plague is an infestation of creatures from "The Void," into the Gargoyle realm of Ter Mur. The creatures that are part of the Plague are commonly referred to as Void Creatures, or Void Daemons. In-game NPCs commonly use the latter term despite the fact that it already refers to creatures which were part of the Magincia Invasion.
It is not yet known if the titular "Void" refers to the Ethereal Void or some kind of hitherto-unknown demonic realm.
Void Plague Creatures spawn first as a slime-like entity and then interact with their environment in various ways, generally increasing in strength and ability to damage. They also tend to draw closer and closer to the Royal City of Ter Mur, sometimes even breaching its gates and entering the city proper. This proves problematic, as they sometimes turn invulnerable when inside Ter Mur city proper, and must be led out if they are to be slain.
Killing the creatures of the Void Plague is a good way to curry favor with Zhah, the Gargoyle Queen of Ter Mur.