Shadow was a pitch black colored rat, although it was believed that he could be a weasel. Lean and wiry, Shadow was also an accomplished climber and burglar. His ability as a spy made him very important to Cluny the Scourge, who ordered Shadow to infiltrate Redwall Abbey and steal the tapestry of Martin the Warrior in an attempt to demoralize the Redwallers.
While Shadow did manage to enter the Abbey and gnaw out Martin's picture, he was discovered by Matthias, who sounded the alarm. Shadow made a run for it, but was stopped at the top of the wall by Mr. Fieldmouse and Constance. Shadow managed to wound Mr. Fieldmouse but as he turned on Constance, she accidentally stuck him with her paw too hard, causing Shadow to lose his balance and fall off the wall to the ground below.
Cluny recovered the tapestry at the bottom of the wall and ran for it, while the abandoned Shadow died shortly after. He lived only long enough to tell (quite calmly) Matthias that Cluny had Martin's tapestry. It was the last thing he ever said.