The Black Cubes are five ancient weapons of mass destruction created millions of Terran years ago to serve as weapons between several of the first sentient species to inhabit the Milky Way Galaxy. Only five of these weapons were created or have been encountered so far by Imperial forces.
Whatever their original origin, the Black Cubes were Chaos artefacts whose powers could only be activated through the use of a blood ritual. Once initiated, the weapons' activation phase typically took a standard Terran day to complete and the device could not be deactivated due to the dangerous build-up of potent Warp energies.
A Black Cube possessed the power to completely transform a target world's climate, transmuting its atmosphere into toxic fumes that were inimical to all forms of life, unleashing horrific cyclonic winds that could rip apart any solid structure above ground or increasing the ambient heat to the point that all of a planet's oceans would boil.
In short, a Black Cube was a weapon of pure planetary destruction that could scour all life from a target world. During the Great Crusade, during the Imperial Compliance of the world of Nurth, the Chaos-tainted human rebel group known as the Echvehnurth, who were holding off the elite Imperial Army forces of Lord Commander Teng Namatjira, possessed a single Black Cube and chose to unleash its powers in a final, suicidal attack.
They felt they had no other choice as the Imperial forces were bolstered by the addition of the powerful Astartes of the Alpha Legion, who quickly moved the world towards Imperial Compliance. Unwilling to be conquered, the Nurthene rebels unleashed the power of a Black Cube during their final offensive against the Imperials and used their own deaths as the sacrifice required to activate the weapon. The resulting destruction transformed Nurth into a Dead World uninhabitable to either its inhabitants or the Imperium.
- Legion (Novel) by Dan Abnett