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 Tobia'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...
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...
I read these books when I was in late elementary school through 7th grade. I am posting all of them which I own. When I was younger I really enjoyed them.
I would recommend reading earlier Animorphs books before this one, too much background to the story.