Well, actually...Dark magic as used by Mar's universe isn't all bad and isn't all good. It's just easier to use it in 'bad' ways because the magic consumes the user over a long period of time. Eventually, there will be nothing left of the user but a weird Dark magic "echo" of who they were. This echo behaves much like the dead person, although emotions tend to be more exaggerated. Case in point: Mar himself, normally.
It's controlled by the user's emotions, and is much more like Lord of the Rings trilogy sorcery than book-learning magic. In theory you could use any spell you can make up...assuming you have enough concentration to actually make the magic 'tendrils' do that. But you'd better be prepared to pay the toll...so to speak....>:3