All books using the keywords: P.J. Vierzba & Z.B. Hurdle

while the Yeerk war on Earth continues, a Controller gets his hands on a piece of morphing technology and goes rogue, and people around town start going missing! The Animorphs (and Ax!) have to track him down and stop him, but he’s pushing the boundaries of both Yeerk abilities and Andalite technology, becoming smarter and stronger all the time. The Animorphs aren’t alone in this fight! The Yeerks want to take the morphing technology from him, and even the Ellimist and Crayak want to stop him before he becomes powerful enough to challenge even them! Can they stop this new threat in time before the Earth and even the galaxy are damaged irreparably?