Book of Magic — John Peel

In the third book, the young magic-users travel to Dondar, a beautiful land of purple skies and blue grass. It is also home to Garonath, the unfriendly wizard of forms. With help from their unicorn friends, they fight his magical monsters, but they soon fall into the hands of Sarman, the evil magician who wants to rule the Diadem . . . and needs their souls to accomplish his despicable mission! But the ultimate battle lies ahead when they must face the Triad and discover what the mysterious Diadem rulers have been planning all along.