Blood is a fell path of Magic. A Sorcerous path in previous games, the ancient stigma against Blood has grown so great that it has been cast off, though the amoral still dabble in its ways.

Blood magic is a series of exchanges, trading in the vital forces of living beings. Thus, when the great mass of life swells in size, there is (in theory) more to trade with the blood-starved forces of the Lower Planes. Unfortunately for Blood Mages, the low and shattered forces are not as starved as their titles would suggest; they are very picky, accepting only virgins of most-pristine quality.

Most spells in the Pantokrator-forbidden school of Blood Magic are rituals; the path is just more suited for lab-work, given the careful prices and procedures for various goods and services.

The world is not particularly kind to Blood magic, due to multiple bans in olden times, so the prices and the required skill needed to get through the Spheres is quite high by arcane standards. It can be argued that the price is paid solely by those exsanguinated and carved up, but this ignores the logistical nightmare of gathering virgins and attracting unwanted attention.

On that note, you can count the number of Blood spells that don't require Blood Slaves on one hand.

A bunch of Blood spells and rituals require another path, targeting the various Lower Planes (or even the spaces between the Spheres), or stirring darker aspects of the paths with dark bargains or corrupting power. The Lower Planes are in conflict with each other, with only the Abyss or deeper realms accessible with just "pure" Blood; lest someone wants to pull an insignificant Imp, a Demon Lord, a careful balance from multiple realms, or an entire war from Hell.

• Blood/Fire allows contact with the Inferno, home of Hellfire, Fiery Imps, and Devils. It also allows for banishment there, albeit with mastery of Blood and high levels of research.
• Blood/Air allows contact with the Tempest, home of Storm Demons and Pazuzu's foul winds.
• Blood/Water allows contact with Kokytos, home of Frost Fiends. It also allows for banishment there, albeit with mastery of Blood and high levels of research.
• Blood/Earth allows contact with the Kingdom of Hell, where dukes lead their Demon Knights to war with each other. A Blood/Earth Mage can also raise Illearth from the bloodstained ground, and vent Infernal Fumes upon distant provinces.
• Blood/Astral allows Horrors to be called and sent, from some sort of Void, but they are not controlled by the summoner. It also allows for sinister Astral Corruption, and the staining of the Moon.
• Blood/Death allows for Vampires to be made, and lets dead Demons known as Bone Devils be summoned.
• Blood/Nature allows the promotion of riotous profusion in the world, even profusion between different species, and lets Dark Vines be summoned. A Blood/Nature mage can also give Poison Resistance to their entire army.
• Blood/Glamour allows contact with Succubi and Incubi, dark Seducers who spawn Cambion Progeny when they succeed. Blood/Glamour mages can also steal fellows away more effectively than regular Blood mages.

A mage with 3 has 5 additional Hit Points. A mage with 4 or higher has 10 additional Hit Points (instead). This is presumably because they have better command over their own life force.

Blood Mages are better at Blood Hunting, too. The base chance to get anything out of a Blood Hunt is 10% plus the mage's Blood level, and one additional Blood Slave is found on a successful hunt with each level.

Last but not least, every level of Blood skill gives a mage 10 Undead Leadership and 10 Magic Leadership; the former lets them command Demons as well, while the latter is just nice to have.

There are eleven different Blood-boosting items, but 5 are artifacts (one-of-a-kind and requiring very high Construction research), two are incredibly difficult to forge, and two of the remaining four items fill the same item slot. The easiest Blood booster to acquire is the Blood Thorn, at 3 and with Level 7 Research; the Armor of Souls (5), Brazen Vessel (5), and Armor of Twisting Thorns (32) are all at Level 5 Research.