An off-hand dragon longsword is the off-hand equivalent to the dragon longsword. It should be noted that if you wield this weapon you cannot equip a shield, since the off-hand dragon longsword is wielded in the shield spot. If you are dual wielding and do not have a shield equipped, there are some abilities that are not accessible to you. This is made up for by the abilities that require you to be dual-wielding melee weapons in order to perform them. The off-hand dragon longsword, like the regular dragon longsword, requires completion of the Lost City quest in order to be equipped.