You crave the taste of blood. Moreover, you need to drink blood to survive.
Drawback
You must drain a pint of blood from a living creature once every 24 hours. Doing so is an attack action, and you can only drain blood from a willing, helpless, or dying (but not dead) creature. The bitten creature takes normal damage from the bite attack plus an extra 1d6 points of damage from the blood loss.
If you go 24 hours without consuming blood, you take 1d4 points of Constitution damage. Drinking a pint of blood cures the ability damage caused by blood deprivation at a rate of one point per round. Ability damage caused by blood deprivation cannot be restored through natural or magical healing.
Special
You must have a bite attack to take this drawback