Power refers to that wielded by the gods.
Any act or thought committed in the name of a god brings some measure of power to that god.
Blood is an especially powerful offering to a god.
“Blood carries the thought. Blood remembers the deed. Blood carries, Blood remembers.” – The Sanguine Catechism
While blood is central to the worship of these gods, most followers offer their own in the form of a drop of blood extracted from the thumb. Human sacrifice is rare and generally frowned upon. One form of human sacrifice that is ‘approved’ is that of blood spilled while fighting on a holy crusade. The concentrated prayer, emotion, thought and act involved in the spilling of human blood in the name of the divine brings much power to the divine.