How do you get a blackwater sword in Oblivion

In the imperial City go to the port and go to the boat inn and buy a room and sleep. then when u wake up u get a quest added and there is pirates on the ship kill them and one of them will have a blackwater sword kill them and get it then go back to bed and wake up and everything is fine and get off the boat. NOW U HAVE A BLACKWATER SWORD!!! THATS VERY SIMPLE !!