To understand blood calling you have to under stand anima/maw
Anima and the Maw: The existence and society of the Sangnir revolve around Anima - and for good reason, for without it they are destined to wither and die. Created from blood and distilled life force, this strange substance flows through a Sangnir's veins, and originates from their "Maw", the reservoir of life-force strongly associated with their hearts, the only organ even the most inhumanly potent of ancient Sangnir are unable to survive without.
and now we get into blood calling
Blood calling/life weaving: In a millennia past most Sangnir Force Users belonged to a tradition of their own, that of the Bloodcallers, also known as the Lifeweavers. Some of their teachings yet remain in the traditions of the Sangnir, but much has been forgotten but for time-worn tablets in ancient ruins and the slumbering minds of ancient Sangnir. They appear to have favoured something resembling Alchemy (with a focus on biological manipulation) and the manipulation of Anima in various ways. focus on biological manipulation) and the manipulation of Anima in various ways.
Sangnir have the ability to drain the life force of others through their bite, using the victim's blood as the medium. Known as Anima, this strange substance has a wide range of uses, including sustaining their immortality and mending wounds, fuelling brief surges of supernatural power, or even powering the esoteric technologies of their long-dead civilisation.
