The Last Battle (The Chronicles of Narnia, Book 7) by C. S. Lewis
The last battle is the greatest of all battles
During the last days of Narnia, the land faces its fiercest challenge - not an invader from without but an enemy from within. Lies and treachery have taken root, and only the king and a small band of loyal followers can prevent the destruction of all they hold dear in this, the magnificent ending to The Chronicles of Narnia.