The Pearls of Lutra (Redwall, Book 9)

When Ublaz Mad-Eyes sets out on a mission to steal Tears of All Oceans, a set of six spherical rose-pink pearls, from the Lutra otter clan, Grath Longfletch must stand up to the evil pine marten while the Redwallers search for the missing treasures. Tansy, a young hedgehog living at Redwall Abbey, attempts to solve the riddle of the missing pearls while at the same time the evil marten, Mad Eyes, desires them for himself.