Like it says in the title. When you hit someone with that sword, instead of the normal fire animation, it shows the target burning with black flame (which makes sense), but the DOT effect doesn't trigger (which doesn't!) Well, I think it should. That's it, that's the request - have the sword's basic attacks apply the black flame DOT effect from the other spells and attacks that use it. I also realize how powerful that could be, so maybe reducing the base power of the sword would be good to not just make it too broken, maybe as an extra option.
