Shadowlith

Alistair the Fourth was a mighty general from a long-forgotten war. Four hundred years later, Alster Lightbridge is barely a shadow of his legendary namesake. Crippled and confined to his family's estate, he spends his days dreaming of something larger than himself—something worthy of his name. 
Encouraged by his tutor to explore the Lightbridge archive, Alster discovers a magical dagger with the power to cleave shadows from their bodies. Blade in hand, Alster finally understands his purpose, though he needs to find a four-centuries-old grave in order to fulfill it.