The Blood of the Light

The Flow comes easily to those who follow the Path of the Light. Anno and the Risen send the Flow freely to those that follow the rules and the Light, but they also work hard to keep the Flow out of the hands of the Dark Ones.
Xi , trapped in The Lower Lands at the Post of Hope does not have the strength to send the Flow to her followers as easily as those that are in the Light. She can pass the Flow to her children, the Fallen, more easily, but they too have difficulty sharing that connection with the Dark Ones.
However, there is a work around. The blood of the Children of the Light, holds a connection to the Flow, and through sacrificing them, and collecting and utilizing their blood, the Dark Ones can break through the barriers that the Risen have put in place and connect with the Dark Flow sent up from the Lower Lands by the Fallen. The amount of blood needed increases based on the amount of Flow that is needed. Simple cantrips need only a small amount, such as a pin prick might gather, while mighty spells might require dozens of the innocent to be sacrificed.

Properties

Physical & Chemical Properties

The blood loses its connection to the flow the longer it is outside of the body. Generally after three days, or if it has dried before then it is no longer useful for the Dark Ones purposes. it is rumored tat the blood of Fey creatures is more potent, and can last longer than three days is stored properly in an airtight container.

Origin & Source

Not all blood will work for the Dark Ones purposes.  The blood must be from a Child of the Light, although age does not matter, but those that have strayed from the Light and not returned are not as potent.

Life & Expiration

The blood must be relatively fresh for the Dark Ones to gain any benefit from it. Blood that is older than three days, or that has dried out due to exposure to the air is not strong enough in the Flow to be useful.