Balacgos's Bane is a cerulean cube created by the ancient blue dragon, Balacgos. Balacgos created the cube to deal with the unharnessed latent magic energy found throughout Azeroth, in an effort to keep it out of the hands of undisciplined practitioners of the arcane. The cube was designed to absorb such magic and hold it in reserve till a time the blue dragonflight had need of it. But Balacgos miscalculated and upon activating the device it absorbed his magic, leaving the dragon a lifeless husk.
The cube was passed on to Neltharion by Malygos to keep safe deep within the earth. For over ten thousand years, the cube remained hidden until discovered by Sintharia of the black dragonflight, who, like the rest of her flight, had gone mad and abandoned their duties as protectors of Azeroth. Sintharia attempted to use the cube in conjunction with the shards of the Demon Soul in an effort to create a new terrifying variation of dragon.
Ultimately, her plans were foiled by Korialstrasz and his allies, and both Balacgos's Bane and the shards of the Demon Soul destroyed one another.