Spells of various sorts that rely on a person's life force, instead of regular mana. Zorian's reading of stolen cultist texts has identified three ways this magic is used.
One way involves tapping your own life force to cast spells. This allows you to cast extra magic at the cost of extreme health losses. Zorian compares it favorably to drawing on un-assimilated ambient mana, at least for a time traveller; it's more likely to be fatal, but will have no impact on sanity.
A second usage is killing other people to extract and use their life force. This can be bargained away with demons or used in special rituals, since life force is very potent, but it's quite difficult to shape someone else's life force directly.
A third usage is in enhancement rituals, allowing people to gain permanent magical abilities at the cost of permanently occupying a portion of their mana reserves. Enhancement rituals do not have to involve blood magic, but incorporating it allows them to be better integrated into the target, reducing the cost and potentially turning them into inheritable bloodlines.