Black blood

Black blood is a chillingly cold black liquid found only in the vault of Orv after which it is named, the Land of Black Blood. It seeps from a dozen locations, forming foul streams that flow into the dark lake known as the Caltherium.

For some reason, aberrations and undead are immune to the cold produced by black blood.

The blood has strange magical properties. If removed from its source, it can be used as unholy water. If drunk, it enhances any necromantic magic subsequently cast – although drinking it in the first place might prove fatal. The blood remains potent for only an hour after being removed, but many of those interested in its properties are likely to have magical means of preserving it.

Those few who are able to decipher a portion of the runes carved on the Cube of Writhing Glyphs find for a while that the benefits they gain from drinking the black blood have increased.

Black-Blooded Creatures

Exposure to black blood – either through living near concentrations of black blood in Orv, or through deliberate (and cruel) experiments – has the potential to mutate most types of living creature. These abominations gain increased physical prowess and self-confidence, but suffer reduced intelligence. They may also be afflicted by numerous mutations.

Such creatures gain the ability to survive in water (and other liquids) as easily as on land, find their chilling physical attacks deal additional damage due to the cold, and they can also exhale a freezing cone of black blood that can fatally freeze those caught within it. Perhaps fortunately for other creatures, the black-blood slowly kills all but the toughest of hosts.

One of the most ferocious of the black-blooded abominations is the creature known as the Moldering Emperor. It possesses sufficient fortitude to be able to shrug off the life-draining effects of the black blood, so there is no chance of it simply dying from it effect.

Ecology

Tribes of cloakers, driders, dero, amphibious humanoids (the charda), exiled fiends and genies are known to harvest the Black Blood to sell to ghouls, liches, and necromancers.

Strangely, the charda are immune to the effects of black blood. As a result, they are able to use it in combat, storing it in their mouth pouches and spitting it at their foes. In fact, they also benefit from their proximity to the black blood; charda fight better when either they or their foes are in contact with black blood, and their elders may develop the ability to replicate certain spells from long proximity to the substance.

References

For additional as-yet unincorporated sources about this subject, see the Meta page.