Dark magic is a type of magic, which draws upon the necromantic energies of Death.
To see a list of other forms of magic and magical abilities, see here.
The Source of Magic Edit
The River of Death has always been the natural pathway of those who have died to pass onto the Astral Plane since time's beginning. For those, however, who fight against the River's current and make the return to Life, they become abominations against the forces of Nature, becoming mangled beings of what they once were: the Dead. As truly dead souls inhabiting the decayed corpses of their old selves, the Dead emanate a pure unnatural energy. This energy can be drawn upon to cast powerful magics, resulting in a malicious, necromantic power called dark magic.
Dark magic is an extremely rare yet powerful type of magic, which can only be wielded by a direct connection to or presence of the Dead. For the Dead to return through the Gates of Death is a feat enough, and the Dead that do make the return to the Living must be strong enough to make it through. As a result, these Dead are often incredibly powerful, emanating a strong energy that renders dark magic near unfathomable in power.
This type of magic however also relies on a constant source, which the Dead can only provide so much of before they become entirely drained and consequently return to the River of Death. Thus, practitioners of dark magic must continue to raise enough Dead to fuel their abilities, which unsurprisingly resulted in the practice itself being forbidden in many countries of the world.