Harmony - Troop Symmetry

It's impossible to implement an effect that only targets undead creatures. You could implement a system where only a Necromancer has access to this spell, but it's not a given that a Necromancer will only use undead creatures, right? What about mixing troops or hiring a Necromancer from another town?
Besides, vampirism is also a dark analogue of dispel. It's no coincidence that it has an additional effect: "becomes undead."

And how can you make it accessible only to a Necromancer? - for example, by linking it to a skill and giving access to this skill only to Necromancers; then, when encountering a hero with dark magic and the "scholar" skill, he will be able to teach them this spell - but that's not the way to do it.
...so how do you do it? - there's the Banshee's Cry spell; it can't be learned, just like Prayer and Inspiration; how does that work?