[Kereth]

Depending on the game system, Necromancer, as the "anti-healer" category, could be anything from an inflicter of blight and disease, crippling and either slowly killing or just weakening an opponent, or even taking the "anti-heal" concept more literally, and performing some effects that prevent healing entirely in the target.

Now, interesting variants on necromancers I've seen in the past often include the negative energy aspect, which can be great or utterly silly, depending on how it's employed, and of course, there are tons which just rely on the classic "I have a huge hoard of undead" motif, so those aren't to be ruled out, but I find myself favoring the "master of disease/decay" approach whenever I've been involved in game design, with an occasional "death energy" mix-in, if you can make it creepy enough.

The only other idea that comes to mind is that necromancy originally stems from a practice of calling forth the spirits of the dead for purposes of communication, rather than as revenant-style entities to be used in combat. Perhaps some flavor could be drawn from that direction?

Just brain storming here, as the flavor itself is probably secondary to the way it's employed.