Capture Doctor Kolher's Orders at Deathspeaker's Watch, then return to Crusader Rhydalla at the Argent Tournament Grounds.
You said the body was taken by cultists? I can only think of one place they would have gone, Deathspeaker's Watch, the encampment on the rise west of the tournament grounds.
Whatever the cultists are doing must be stopped. The rumors I've heard can only be true, then. The infamous Doctor Kohler must be among the cultists, practicing his arts. Look for a rise or an altar overlooking the cultist camp, slay the doctor, and look for proof of his plans. Slow down his work in any other way you can.
Did you find out what they're doing with the Black Knight's remains?
The doctor will not have time to complete his work, but his plan is already in motion. From what you've reported, the Lich King's servants have reanimated and empowered the Black Knight and there's no telling what the knight's master has in mind. You have done well in uncovering this plot. I will tell High Crusader Adelard of your findings and together, we will alert the tournament's participants.
You will receive:
Upon completion of this quest you will gain: