Blood

Blood (BL) is a reagent that is in use on Pagan. The essence of life in creatures, it is not rare per definition, but gathering human blood can be problematic. Either one is nice and asks someone for a donation, or it is gained criminally by way of murder.

Pagan
For the magic school of Necromancy, human blood was needed for spells of summoning and communication with the dead. For each spell, blood was needed in exact quantity of one vial. Note that it explicitely is human blood, other sorts of blood like the powerful dragon blood did not work with it.

Trivia

 * That blood is a reagent in Necromancy plays into the wide-spread misconception that satanic magic (on which many parts of the magic system in Pagan were based on) uses human blood for its rituals. This is wide-spread trope in many RPGs.