A black sun is an extremely rare meteorological event in the Manifold Sky characterized by the 'eclipse' of a Cube layer when viewed from the adjacent cube layer. While it is generally caused by a massive infusion of soot, smoke, or dark dust into the atmosphere of a cube layer, at least one instance of a black sun has occurred due to a layer-spanning bloom of billowing hate spores. This event, known as 'The Horror Dusks' in Vale Verdial records, was observed over the course of three days in early Iksunten of the year 9978. Multiple research operations in the affected region (namely Distal B cube) were lost to anarchy as the psychoactive spore clouds caused the scientists there to go berserk. Survivors of The Horror Dusks were difficult to rescue due to the need to carry out rescue operations with strict quarantine and sterilization protocols to prevent any spores from spreading to Petalcap Vale in the adjacent Caudal D layer; as of three weeks after the event, many scientists caught in the disaster remained unaccounted for. While a black sun generally indicates a significant environmental hazard for any caught on the effected cube layer, they are merely perceived as ill omens for those on the adjacent layer; except in the case of volcanic eruptions, particulate is seldom able to cross an inflection layer due to the extreme altitude involved.
A black sun is caused by the reflected light of a cube layer - along with much of that cube layer's ambient light - becoming occluded by airborne particulate. As a result of this occlusion, the layer appears to be darkened or, in extreme cases, may even become black when viewed from the adjacent layer. Onlookers on the adjacent layer see the affected layer as a dark spot in the sky surrounded by a 'corona' of light emanated from altitudes above the rolling clouds of particulate. Over time, the pollution either falls back to the ground or sinks into the lowlands and commissures of the affected layer, bringing an end to the black sun event.
Most black sun events occur in the Ventral Tesseract (due to volcanism), Eastern Tesseract (due to dust storms), and Distal Tesseract (due to spores and the natural atmospheric composition). Nevertheless, hazardous black suns can be artificially created in any environment in which sufficient combustible material is present, a fact which prompted most Human factions to sign the Medial B Accords to prevent the use of indiscriminate incendiary weapons.