Item from soul-eater by
A madness-amplifying substance developed by Medusa that grants enhanced durability and power at the cost of progressive psychological deterioration and susceptibility to kishin-wavelength influence.
Black blood is Medusa's masterwork: a substance that rewrites the body's relationship to madness from weakness to resonance. It makes the host harder to damage, more powerful, and increasingly susceptible to the madness wavelength that the Kishin radiates. The trade is clean and catastrophic — more power, less you. Crona carries it in full concentration. Soul carries a smaller infection. Both feel it differently: for Crona it's the medium they've always swum in; for Soul it's a voice through a wall, present but not overwhelming — yet. The little demon in Soul's mindscape is its emissary.
A dark, viscous substance that moves with slightly too much intention — it pools and flows in ways that suggest awareness. When dormant in a host body it is invisible. When activated it rises through the skin or hardens across the body's surface like dark armor. It does not look like blood. It looks like something that decided to be blood.
Also known as: Black Blood, Black-Blood Madness Amplifier