The Path of Blood can be found on pp. 178-180 of Vampire: The Masquerade.
With a taste of the subject’s blood, the vampire learns the following information: Whether the subject is vampire, what clan he belongs to (within the thaumaturge’s experience — if he’s never encountered the clan before, the information he gains is “new clan”), the subject’s Blood Potency, and whether the subject has committed diablerie in the last month.
The thaumaturge must touch the target (this action is considered reflexive; see Touching an Opponent on p. 157 of the World of Darkness Rulebook) before using this power. The character can force the character to expend one point of Vitae per success, and may exceed the usual limits imposed by Blood Potency. The thaumaturge can force the target to spend Vitae on anything he could normally spend it on, including Physical Augmentation and counterfeiting life. Each success also imposes a –1 penalty on the target’s roll to resist frenzy for the rest of the scene.
The character raises his effective Blood Potency. Additional successes can be used to raise Blood Potency or to increase the duration by one hour: two successes on the roll can raise the character’s Blood Potency by one for two hours or by two for one hour (the first success in each category is “free,” provided the Thaumaturgy roll succeeds). Increasing Blood Potency increases the vampire’s Vitae pool and adds to resistance rolls for certain powers. It may also increase the amount of Vitae the character can expend in a turn. Finally, the character responds to other vampires differently with regards to the Predator’s Taint, based on his new effective Blood Potency. When the power wears off, any Vitae in addition to the character’s normal pool is lost.
Subtract the target’s Defense from the roll in addition to the usual –4 penalty. Each success drains one point of Vitae from any visible target within 50 feet of the vampire. The blood erupts from the target’s mouth and nose and flies toward the vampire, who absorbs it into his hand or mouth. Used on a vampire, this can create a Vinculum.
This power requires that the vampire touch the target (see p. 157 of the World of Darkness Rulebook). The touch is a reflexive action. Subtract the target’s Stamina from the spell, in addition to the usual –5 penalty. The character then boils one point of blood per success on the roll. Any success kills a mortal immediately; vampires suffer successes as aggravated damage.