Blood Weapon is the Dark Knight special two-hour ability in Final Fantasy XI. As with all two-hour abilities, it is usable from level 1 up. The ability, when activated, will give the Dark Knight a 30 second special ability status effect.
While in effect, every successful landed hit by the Dark Knight will drain a significant additional amount of HP. Blood Weapon has absolutely no effect on accuracy so it is completely possible that a Dark Knight could miss every single attack he has during that 30 seconds and gain nothing from using the ability. Also, like all HP and MP siphoning abilities, this has absolutely no effect when used against undead.
Template:Final Fantasy XI abilities