Dark Magic

Dark Magic is powered by dark corrupted souls.
Spiritual energy has to be siphoned, often painfully, off of others, or souls can be destroyed generating immense power. This disregard for life makes this magic taboo. It's the dark counterpart to White Magic which uses the caster's own spiritual energy for augmentation and healing.
Red Feysite is used as a "lens" to focus spiritual energy to amplify spells. This magic is addictive and continous use erodes the soul resulting in blackened fingers and tainted veins.
It comes in several varieties;
Necromancy - used to create ghouls and summon phantasms.
Blood Magic - using the latent magic in blood, combined with geometric patterns, to create unspeakable horrors and warp reality.
Eldritch Magic - making pacts with the Elder Gods, usually at the cost of the caster's sanity.

Manifestation

It's a dripping sickly green energy that forms tendrils

Localization

Found in all sentient life, from men to elves

Type

Metaphysical, Astral