Giant's Blood is a vampiric artifact.
This is exactly what it reads - giants’ blood. In the ancient days when the giants walked the earth, only foolhardy vampires would prey upon these loathsome creatures. A few vampires did, however, and they made sure they bottled the remainder of the prized blood that they could not consume immediately. The blood, at least temporarily, and possibly permanently, increases vampire’s Potence score dramatically.
Players Guide to the Sabbat, p. 153-154