Outcast of Redwall by Brian Jacques

Abandoned as an infant by his father, the evil warlord Swartt Sixclaw, Veil is raised by the kindhearted Bryony.  Despite concerns from everyone at Redwall, Bryony is convinced that Veil's goodness will prevail.  But when veil commits a crime that is unforgivable, he is banished from the abbey forever.  Then Swartt and his hordes of searats and vermin attack Redwall, and Veil has to decide: Should he join Swartt in battle against the only creature who has ever loved him?  Or should he turn his back on his true father?