Shadowlith - Umbral Blade, #1

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.