Black magic is a very powerful tool. Its forces are as great as they are impenetrable. This art can change people's lives, even the caster's, making it for those who know (or don't know) how to use it; fragile, poisoned or cursed. Darker practice of this art can even breathe life into inanimate object. The caster will then be able to command or summon creatures to target his enemies and tormented their life as he want. It has the power to bring changes by using forces which is beyond senses.
One of the most famous and powerful casters who practice dark magic is the faceless god Sadida himself. His followers; the Tree People also practice dark art but that doesn't mean they are evil. One symbol of their magic is their dolls and curses.
The magic is defined good and bad depending to the caster's intention. You can use black magic to help your allies like Sadidas or use White magic for bad intention.
Everything depend of the caster.