The Animorphs have met the Ellimist. He helped to save the kids when they were about to be eaten by a Taxxon. He helped to free two Hork-Bajir and restored Tobias's morphing ability. But even though the Ellimist has enormous power, he is not all-powerful. He has an enemy. The Crayak.
The Crayak and the Ellimist have decided that a battle is necessary to prove their ultimate power. But they don't intend to fight each other. The Ellimist will choose the Animorphs and Erek the Chee as his champions; the Crayak will choose his own army. If the Animorphs lose, they'll be erased from the universe altogether. And without any Animorphs, there'll be no one left to fight the Yeerks...