The Great Battle refers to a war that began between the Mystic Wizards and the forces of the Underworld. The Mystic Wizards were led by Leanbow, and their ranks included Daggeron, Udonna, Niella the Gatekeeper, and Calindor. The Underworld forces were led by Morticon, who served as general to The Master. After a series of fierce clashes, the Underworld forces were banished back to their home realm, but at the cost of Niella's life and Leanbow being trapped with them. Additionally, Calindor betrayed the mystics and attempted to kidnap Leanbow and Udonna's son Bowen, but ended up cursed and trapped inside of a cave with Daggeron. Of the other Mystic Wizards, Udonna was apparently the only survivor of the war.
Years later, the Great Battle would resume when Leanbow, brainwashed into becoming Koragg the Knight Wolf, escaped the Underworld and led a bid to free the Master and his forces. This would be challenged by Udonna and by the successors of the Mystic Wizards, the Mystic Force Rangers. Though Udonna lost her Snow Staff early in the renewed conflict, she continued to mentor the Rangers and help where she could. Morticon managed to return to the surface after Niella's daughter Clare was discovered to be the new Gatekeeper, but was destroyed by the Rangers. Shortly thereafter, Calindor and Daggeron were freed from the cave, with the mummified Calindor being transformed into Imperious, Morticon's replacement. Daggeron, cursed into becoming a frog, would later be returned to normal and join the battle as the Solaris Knight.
Following Imperious' demise and Leanbow regaining his former identity, the battle would be carried on by the Ten Terrors. The Rangers and Leanbow battled them for some time, even after the Master was revived in a new body. His attacks led to them briefly losing their powers, but they were rejuvenated by the people of Briarwood and the mystical world, giving them the strength to overload the Master and destroy him. With two of the Terrors and Necrolai redeemed and the rest of the Underworld army destroyed, the Great Battle came to an end at long last.