This is the second and final quest of the Foul Magics quest chain. You will find Marez Cowl, the holder of the Befouled Bloodstone Orb, in Stromgarde Keep in either of two locations - both within the walls of Stromgarde, but outside of the actual Keep and to the west. To find Marez, take a left below or at the top of the ramp (the one located straight from the entrance), and you'll find her hiding behind a building in either location. You will be able to hear her channeling some sort of spell.