The Shadow God was the lord of the Plane of Shadows. When Tobijah Kruel ascended to the form of a Void God, he purposely stole power from the Shadow God to be able to usurp his throne. He was cast out of his realm and thrown into Azeroth, where he met Warren Greystone, to whom he explained the circumstances, albeit in a rather cryptic manner. When Kruel collapsed the floor of Ginchar Crater the Usurped, as he called himself, saved Warren and transported him to Ginutzim, where the wedding of Linus Wrynn and Arinre Greymane was taking place. Warren interrupted the wedding, in the process destroying a Kruelist trinket which had been poisoning Wrynn's mind. The important figures made their way out of Ginutzim before it was annihilated by the Sanctions of Kruel.
Meanwhile, the Usurped had been studying the situation with Kruel, and discovered a weakness. To ascend Kruel had absorbed countless life-forces from those within Ginchar, amongst them Minerva von Xie. However, Minerva also carried within her the taint of the Other, the strange entity created from the essences of Xostheron and Akaerna-Sagai, as had been seen with Hellen von Xie. When Minerva had been absorbed Kruel had accidentally absorbed the Other as well, which created an irremovable corruption within him. He waited with Warren for the time when the rift between the Plane of Shadows and Azeroth would expand far enough for the protective pocket it provided to dissipate, allowing his weakness to be exploited.
When the rift grew wide enough, the Usurped finally acted. He forced the Void God completely into the Shadow Realm and sealed it, sacrificing himself in the process.
Even with his throne usurped the Shadow God had great powers at his disposal. He was able to move himself and others great distances instantaneously. However, he was unable to restrain the flow of his energies entirely, causing a distortion of his voice and the draining of the memory of mortals.