Exactly what does Black duelist do? It says makes spells work better in head-on combat, but what exactly is head-on? Does that mean when you're tanking, or just being in combat at all?

Also, would you say it was an absolute must have? I've seen in alot of PBF's that it was chosen by most of the stronger necros, but I wanted to see if anyone had any complaints.

Also, about how expensive is it? If you could relate it to another edge in terms of edge-points, that would be extremely helpful.

Thanks in advance.