Hull's Long Sword is found in a chest in the far corner (646, 293) of the Candlekeep Barracks. It is needed to complete the side quest Hull's Sword. This weapon is identical to a normal Long Sword in every way (thus still breakable because of the Iron Crisis), except that it is the only sword that will satisfy Hull and his quest. You could keep it and wield it yourself, if you wanted to, or you could sell it to Winthrop for 5Image:Goldpieces.png (give or take, depending on your Charisma score) instead of returning it to Hull for a much better reward, if you've got some sort of self-punishing grudge against him.