Animorphs

Jake and his friends are given the power to morph into animals by a dying alien who tells them the Earth is under attack by the invading Yeerks, and they are called upon to test their new abilities almost immediately when they learn that Vissar Three, deadliest of the Yeerks, is after them.

Tobias has stayed in his morph too long and must now live out the rest of his life as a hawk, but that does not keep him from contacting his friends and continuing the fight against the Yeerks, a race of hostile aliens that are invading and controlling the brains of humans.

Rachel and her friends run into a dying alien who gives them the power to morph into any animal they touch, placing the responsibility of saving the Earth from a silent, deadly attack completely in their hands.

The Animorphs meet the Ellimist. He helps save the kids when they were about to be eaten by the Taxxon.