Arterial Strike( Complete Warrior, p. 96)
Your sneak attacks target large blood vessels, leaving wounds that cause massive blood loss.
base attack bonus +4, Sneak attack ability,
If you hit with a sneak attack, you may choose to forgo +1d6 of extra sneak attack damage to deliver a wound that won't stop bleeding. Each wound caused in this manner saps an extra 1 point of damage per round from the victim, until the victim receives the benefit of a DC 15 Heal check or any cure spell or other magical healing. Wounds from multiple arterial strikes result in cumulative bleeding loss (two successful arterial strikes cause an extra 2 points of damage per round until healed). You may deliver only one bleeding wound per successful sneak attack.