Brian Jacques: Mattimeo

Mattimeo, the son of the Warrior of Redwall Matthias, is stolen from the abbey by an old foe of Matthias', a young rat better known as Slagar. anyway Mattimeo is taken to the sunken home of Malkariss, an infamous beast who rules over a huge under-ground kingdom. Matthias knows nothing of this and sets out with his crew to rescue his son and all the others that have fallen to the monster.

REDWALL (again)