I would recommend reading earlier Animorphs books before this one, too much background to the story.
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.
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...
A fairly good story in the series. Introduces another race that really makes this series feel even more like the "Stargate" television series.