The Amber Spyglass

In the astonishing finale to the His Dark Materials trilogy, Lyra and Will are in unspeakable danger. With help from Iorek Byrnison the armored bear and two tiny Gallivespian spies, they must journey to a dank and gray-lit world where no living soul has ever gone. All the while, Dr. Mary Malone builds a magnificent Amber Spyglass. An assassin hunts her down, and Lord Asriel, with a troop of shining angels, fights his mighty rebellion, in a battle of strange allies—and shocking sacrifice.

As war rages and Dust drains from the sky, the fate of the living—and the dead—finally comes to depend on two children and the simple truth of one simple story.

Synopsis from Open Library.