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 overpower Mr.Fieldmouse but as he turned to Constance she accidentally stuck him with her paw too hard,causing Shadow to lose his balance and fall screaming 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 Matthias (quite calmly) that Cluny had Martin's tapestry. It was the last thing he ever said.