The Attack (Animorphs #26)
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.
So, the Crayak and the Ellimist decide that a battle will prove their ultimate power. But they don't intend to fight each other. The Ellimist will choose the Animorphs, Ax, and Erek, the Chee; the Crayak will choose his own army. If the Animorphs lose they will be erased from the universe altogether. And there'll be no one left to fight the Yeerks....