Arterial Strike

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.

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.

Your sneak attacks target large blood vessels, leaving wounds that cause massive blood loss.