Never used it myself, but the way I understand it, Necromancy adds books and recipes for craftables that can be activated to implant them into non-pulped zed corpses, and possibly even other corpses to revive them as friendlies.
The craftables require somewhat high-skill, and a fair amount of resources, plus a book, to create, and have a chance of backfiring and keeping the revived entities hostile. I’m not sure whether the revival occurs instantaneously, or if you have to wait for it to occur naturally, but I’m willing to bet the former, since you can probably use it on things that don’t normally revive.
I’ve never bothered with it since it requires time and resources to prepare, and only has a chance of making friendly that Telekinetic-Fire-Breathing-Laser-Eyes-Ultra-Murder-Hyper-Hulk that you spent two days trying to kill, and using it on a normal zed is something of a waste.
“Creating a zombie slave” in the vanilla game is just dismembering them. You chop the arms off to stop them from grabbing and punching, though they may still be able to bite. Naturally, this requires a cutting implement, and it incurs a severe morale penalty unless you’re a psychopath, so I don’t usually do it, but I’ve heard tell that you can somehow use them as a packmule, since they’ll follow you relentlessly.
