They wouldn't have to be human zombies. In D:OS you would frequently run into zombie dogs for one thing. They would just be the enemy killed with stats modified into a zombies.
Certainly... although that depends on if Larian wants to make a ton of models of zombie for every possible enemy. And that still wouldn't cover them all, because there can be spiders and plants and beings made of pure elemental energy for which a zombie form would not make sense. (Probably also wouldn't make sense to raise a zombie from a skeletal enemy's corpse, nor to detonate it.)
You give 50% of your max health in HP and perhaps 50% of both armors just to heal 25% of your max health? Why the hell should anybody want to do that? You turn more into less with no real benefit except probably now being two characters halfdeath or even lower. If you would heal for double the amount or at least 75% it would perhaps be worth even thinking about. Or if the target would get healed for several rounds for half of the sacrificed HP.
Also I think it should work on current health and not max health.
I'm open to tweaking the numbers.
Okay we can lose the penalty to armor. That's fine with me. It was really mostly in there because people complain that "CON means nothing because only armor means something - no armor and you die".
The idea is that it allows the Necromancer to provide healing to an ally as part of the Necromancer skill set of Life Manipulation. This can be useful for a backline combatant like the necromancer who is less likely to be injured to support a front-line combatant who is injured.
The numbers can be tweaked sure. It could be changed to take half the current health of the caster instead of half the maximum health.
So let's posit that it costs half the current hitpoints and restores 3/4 of that amount. Lhose at level 2 has 45 HP. So half that would be 23 HP. 3/4 of 23 HP is 17 HP, which is more than the 13 HP which Lhose's Restoration restores.
It is not as efficient as Restoration, but if the target really needs that 17 HP right now, it'll be a godsend, and the gap between those numbers will only widen over time.