Blade of Purity contains two linked tales of the Grey Knights:

Incorruptible - Grey Knights Justicar Styer and his squad are assigned to locate a lost Grey Knights vessel, the Purifier ship Blade of Purity. Boarding the ship, they find evidence of battle and possible Chaos corruption. Has the impossible happened and Purifiers have fallen to the thrall of the Ruinous Powers? Or is something else stalking the halls of the Blade of Purity?

True Name - Departing from the Sanctus Reach after their ill-fated battle with the daemon Ku'gath Plaguefather, the Grey Knights encounter their nemesis again as they enter the warp. As the battle becomes one between Gared and Ku'gath himself, it falls to the Librarian to save his brothers - and his soul.