Bad unicorn trilogy

Max Spencer is an underachiever but when a carnivorous unicorn, Princess the Destroyer, and an evil wizard, Rezormoor, bring him, the only one who can read the legendary Codex of Infinite Knowability, to their magical realm, he must find the courage to save himself, his friends, and the entire human race.

After saving three worlds, Max has trouble settling back into middle school and life at home, but when he tries to return to the magical planet of Magrus, the evil Maelshadow begins changing humans into monsters in an effort to take over the planet.