Pearls of Lutra (Redwall Series #9)

On the Isle of Sampetra, Emperor Ubla, better known as Mad Eyes, sends his lizard army on a mission to capture Redwall. Meanwhile the inhabitants of Redwall are on a mission of their own--to solve the six fiendishly difficult riddles that will lead them to the rose-colored gems, the Pearls of Lutra. As they get closer to solving to riddles, the lizards get closer to Redwall and a battle to the death begins.