So, I'm running a campaign, and one of my runner's backgrounds involves a blood mage cult, of sorts. I can't find much information on blood mages in the books: it's possible I'm just looking too carelessly, or don't have that book, or whatnot.

I know that they get increased power into their spells by spilling blood as they cast spells, but numbers-wise, how does that come across? What specifically is involved? And do they have to be free mages, or can they be shamans and adepts too?

You'll find info on them in the Threats chapter of MitS.

There are many form of Blood Magic. The most known is the Aztec blood magic, which was used in a scheme to bring about a new world order in which Aztlan would reign (this involved horrors). That failed. Needless to say, Blood Mages are extremely rare since you have to be a high Aztec priest to be taught the secrets, and have a 1 million nuyen bounty on their heads. Other forms of blood magic exist, such as The Great Ghost Dance, but that was a one-time deal, and it wasn't very good for the world. Ancient History's website must have more info.

Although MitS' notation about the derranged serial killer who made his own form of magic and initation opens the floodgate for bizarre magical types that fit no known magical definitions.

-Siege

hey Seige, what were the details of that deranged serial murderer? I don't have my books. Actually, I guess anyone could answer.

And although it's a 2nd ed. book, the Aztlan sourcebook is still a good definition and description as well.

It's a notation written from the perspective of a Detective-Mage and describes in passing the offender's metamagic techniques and initiation based on his convuluted methodology regarding the "Old Ones."

The Detective also notes it seriously disturbed him because it actually worked.

To my mind, that suggests the GM has complete leeway to throw the magical curveball once in a while.

-Siege

@ Siege:

Does that story make you think of another thread you and I post to?

I must make a note to create a page dedicated to blood magic in the near future.

As for your question Llefyn (love the name, by-the-by, very Welsh):

Blood magic is officially codified in SR 3rd Ed. as the Sacrificing metamagic.
Individual magical threats may learn this metamagic from astral quests or certain spirits, but this route woudl drive a PC mad.
NPCs may also learn it by joining certain initiate groups (namely: the Blood Mage Gestalt, the Aztler shamanic Path of the Sun, certain Aiustralian aborigine koradji shamans, the Aleph Society, and certain British and Celtic druidic groups.)

The insane serial-killer everyone's referring to was in Awakenings. A fine story, that, and a good example of an individual magician learning blood magic.

Put simply, blood magic allows you to reduce Drain by inflicting physical wounds-either on yourself, or somebody (willing or unwilling) nearby. Likewise, by sacrificing a living (current rules focus ons entient) creature, you may summon an Essence-draining Blood SPirit (nasty-nasty).

Oops, my bad.

-Siege