Find the Black Guard Swordsmith in Stratholme and destroy him. Recover the Insignia of the Black Guard and return to Seril Scourgebane.
If I am short with you, it is for a reason. The plans for the creation of the great blade, Corruption, have been stolen by the Black Guard: Elite skeletal units of Baron Rivendare.
Yes, Baron Rivendare of Stratholme.
The Black Guard swordsmith now guards those plans with his... undeath.
Slay him and return proof of this deed and I shall grant you the knowledge required to create a blazing rapier!