Into Battle (The Seventh Tower, Book 5 of 6)

Blurb

Into Battle is the fifth book in Garth Nix's The Seventh Tower series, published in 2001 by Scholastic. In the book, the Icecarls are preparing to attack the Chosen's Castle to protect the Veil, which lies vulnerable as a mysterious force of evil begins to reveal itself. In this book, the Icecarls are fed up with the Chosen, and prepare to attack the Castle with War-Chief Milla Talon-Hand at the lead. Milla, trained with a Sunstone, and wielding a talon of Danir, is prepared to defend her army from anything, no matter what the cost.
Milla's Icecarl army quickly destroys and conquers the Red and Orange levels, terrorizing all with a Violet Wave of Destruction, produced by her half of the Violet Keystone- which she thinks is just a normal Sunstone at first, but slowly begins reconsidering.
The cover design and art are by Joan Moloney and Steve Rawlings respectively.