When a mysterious package lands on Louisa Hancock's doorstep, the Philadelphia museum curator can hardly anticipate the nightmare that's about to envelop her. The package is addressed to her father--an expert in Viking culture--and inside is a ninth-century sword, a chilling thank-you note, and photos of two dead bodies in a tableau evoking a Nordic funeral. The gruesome images match a recent crime scene. But before the police can investigate the...