Blood Magic – Oaths and Sacrifice (Encyclopaedia Arcane), softback supplement for D&D 3rd/3.5th edition, by I Sturrock
“The ancient practice of blood magic has been outlawed in many civilised societies, for though it allows access to staggering levels of magical power the price can be very high. Savage tribes often respect blood magic, though even they tend to fear and avoid it where possible- its power is the stuff of legend, to be invoked only by those who expect to achieve great things or die in the attempt. A few warriors of the old school will swear blood brotherhood or other blood oaths but learning much more than that is generally the province only of primitive sorcerers or the most vile and despicable wizards. Those who go into blood magic with good intentions often find themselves seduced by its savage power, and soon want more- at whatever cost, up to and including dozens or hundreds of innocent lives.
The power inherent in sentient blood has been recognised by many of the great philosophers and sages throughout history, both religious and secular. Whether spilling his own blood to fuel his spells, creating a ritual to bond two comrades in blood brotherhood, or sacrificing enemies for magical power, the practitioner of blood magic is drawing upon that ancient, primal force, and must take great care that he can control it, and not vice versa.”