Elfangor's Secret (Animorphs, Megamorphs #3)

Eighteen-year-old Henry, a human-Controller, is a dangerous Earth enemy now that he's discovered Elfangor's Time Matrix. Even worse, until recently Henry was better known as Visser Four! He sees the Time Matrix as his big chance to win back his lost title. The Animorphs have been given the power to pursue Henry through time, but the gift comes with a big price--the life of one of the Animorphs!.