Blood Magic requires the Sacrificing Metamagic, a secret and ill-reputed technique that while not inherently evil, is dangerous in its effects on the Astral plane. If one were to liken mana to life, and vice versa, blood magic would be exchanging lifeforce directly for magical power. The lifeforce, being a more conducive form to human grasp, is easier to manipulate and is used as a substitute for the raw mana taken from the astral plane. Blood magic makes spikes in the background count, as the "exhaust" from the spell returns to the astral plane as converted mana. This phenomenon is mentioned in Canon and one of the grandest uses of Blood Magic, the Great Ghost Dance, was responsible for a mana spike that would bring "the Enemy" into our world.
Blood magic in Shadowrun has two forms. Blood sorcery, and blood summoning. Blood Sorcery is when the magician uses life from a "donor" to reduce the effective force of the spell. For every box of damage that the magician uses from the donor, they reduce the Force rating used for the drain of that spell. This force reduction does not affect the actual spell result, just its drain. So, a spell cast at force 12, by a mage that inflicts 6 boxes of damage from the donor (including themself as the donor, if desired) can reduce the effective force for drain by 6 to 6. Normally, this kind of force spell would do physical damage, however, with the reduced force, it is now stun damage. The reduction is further increased if the target is killed and the drain force is reduced by the targets essence. To use a donor, the magician must harm a person whom is incapable of resisting, the most common way being blood letting, however this is not a requirement, as only harm must be done. Some blood magicians have been known to exhibit the ability to drain lifeforce directly without ritual blood-letting. These people often use the "killing hands" adept power to do so. However, the damage done must be done directly and in close proximity. So no guns or indirect projectiles. Blood Sorcery may be done with any other metamagic, be it ritually or quickened or even anchored.
Blood summoning is also very nasty. To blood summon, a person must be ritually sacrificed for the purpose of summoning that spirit. The force of the spirit summoned is based on the creature sacrificed. Small animals usually only summon force 1 spirits, while larger animals are 2-3. Awakened critters add +1 to the force, so a unicorn would be 4 force. Mundane sentients use their essence rating. Magic users use their magic rating to determine the force. So a Magic 10 mage being sacrificed to summon a Blood spirit would be very, very nasty indeed. Blood spirits are summoned in ritual, taking force hours to summon. Once that is complete, a conjuring test is rolled as per an elemental, against the force with a drain as per a normal spirit. Each success gives one service, so 3 successes = 3 services. Greater form blood spirits may only be summoned if the sacrificed is a powerful awakened critter (a dragon) or an initiate. Use invoking as normal. The spirit's form is usually as a bloody mist cloud, like a blood elemental if you will, or occasionally in other forms. Some powerful spirits are known to take the form of the sacrificed, however twisted and gruesome.
The spirit must stay within force x 100 meters of sentient beings cannot leave this range. Blood spirits lose 1 point of essence per day, and a such, must use the essence drain power to replenish themselves. Greater form blood spirits have the Engulf power, allowing them to drown their target in blood, treat that as a water engulf. Blood spirits can go free, and in doing so, only lose 1 point of essence per week, instead of per day.
There are other more obscure forms of blood magic. Some speculate that the nature of cyberware itself is a form of blood magic. A ritual known as a blood oath, can be sworn that will bond the parties involved in a contract that if broken, releases a curse upon those who break the contract. This can be likened to a materially linked anchored ritual spell. If the condition of the quickened spell is broken, the anchored spell is released through the material link to the target. Since blood is spent in this manner, the box of physical damage used in this ritual does not heal which keeps the spell anchored until the Oath is fufilled.
Ancient forms of blood magic included magical items that activated spells and effects through the use of the owner's own blood. These magics have never been discovered in the scope of the Shadowrun campaign, however.
