The Imperial War and the Supreme War were derivatives of the Powerhouse War. The Supreme War was a larger scale of the Imperial War. It was where outdated supreme-rank forces fought in order to cultivate Supreme Gods. It would be held in an independent realm created by a supreme powerhouse.
In the history of the Realms of Gods, there were only a few instances of the Supreme War, and each time, the outcome was tragic. Since it was a life-or-death fight, if no Supreme Gods emerged, they would have to continue fighting, resulting in great losses. Even if a Supreme God emerged, the cost was still too high. For this reason, very few supreme-rank forces would declare Supreme War on another supreme-rank force.