Blood Weapon is a recurring ability in the Final Fantasy series. Often a form of Limit Break, it grants the user the ability to drain an opponent's HP to heal themselves when attacking, similar to the Blood Sword and other Blood Equipment weapons.
Blood Weapon is the Dark Knight's special two-hour ability in Final Fantasy XI. As with all two-hour abilities, it is usable from level 1 up. Once activated, for the next 30 seconds the Dark Knight gains a special status buff which heals the Dark Knight HP equal to the damage they inflicted each time they successfully land an attack. Like all HP and MP siphoning abilities, this has absolutely no effect when used against the undead.
The Dark Knight can use Sanguine Sword, inflicting Dark-elemental damage to an enemy and absorbing their HP. It has a range of 3 and needs 500 JP to learn. Gaffgarion can use Shadowblade as a Fell Knight, which acts as a stronger version of Sanguine Sword.
Firion's EX Mode turns all his weapons into Blood Weapons, and grants him the Blood Weapon ability. While in EX Mode, all of Firion's HP attacks, including his EX Burst, heal him for HP equal to the HP damage inflicted. The Emperor gains the "Blood Magic" ability in his EX Mode, which functions identically to Blood Weapon.
Firion's exclusive ability is Blood Weapon, and is learned at Level 40. Blood Weapon activates automatically in BMS after the player chains 50 notes, and for the rest of the BMS heals a small amount of HP each time Good or better is achieved on a Trigger.