Null mines were created to punch holes through the Web of Magic. When detonated they would sometimes create a rift between systems or worlds. Because of their inherently unstable core, these devices did not always work. The core's hold unstable energy from Chaos; indistinguishable from some other elemental energy making them hard to find. Some have taken to calling this null energy - since it is not like what it looks like, it may look like elemental fire, but in reality is an energy that destroys other energies around it.
When detonated near a weak spot in the Web of Magic, the source of a world's arcane energy, the mine has a lot to feed upon; enough that it could keep a rift open for thousands of years before creating an area of dead magic. Eventually most rifts are closed by the Web's weave spiders or by those that see them as a serious potential threat to the stability of either side.
In the Creation War, the primordials used null mines to create rifts between Chaos and Nawirrûs. In the Demon Spawn War, caches of null mines were discovered by the demons. Most of these were on the primordial forge worlds of Piranoths Steps. After this system fell to the demons in the Abyssal Release (7777 DE - 8777 DE), these mines were used to create inter-system rifts between the worlds of the Abyss and those of the Quara'tun System.
Since the Dawn Era, null mines have been used to pierce the Web of Magic. These creating rifts, which usually leads to havoc. In the Lith-Crillion Era, the Concubine General led an army from Fimbulwinter through a blizzard-like vortex. This rift was created with a cluster of null mines. They passed through a temporary rift to Bal-Kriav, assailing the peoples of the continent Elemantum.