I assume FantasticDorf allows that all the simulation stuff can be filled in as the magic system is developed. Useful to have this suggestion as a starting point, at least.
I didn't seek to overthink it as anything but a generic ability, it is divine power not solely magic in my own view, just as the dark arts of necromancy is a learnt secret and a power delivered by the gods rather than a sphere reliant exercise for the currrent here and now.
When magic spheres are actually relevant for destruction, the undead can be undone much more fluidly: like with a enormous fireball, but explicitly the priest's suggested ability for the near term of the game is a extrimity that can be expanded upon later and doesn't yet rely on explicit ties.
Simply appointing someone to a priest and -bam- magic wouldn't fit DF imho. Especially after what I've heard from the plans for the magic arc so far.
I mean it's a perfectly reasonable criticism because of the setup, even if the pre-requisites were given more weight, such as to be a holy leader of a church or a true prophet it'd probably deliver at the same result as the paragraph's above conclusion on divine intervention of forcing things deus ex machina versus a realised energy of magic variable with rules and limitations being in the world.
Implicitly all temples are ordained with a minute amount of holy power, as the gods take notice to curse those who defile statues around itself, i wouldn't personally rule out that priests being the custodians of these places have the authority to do a action like exorcism while also from a player's side being a temporary guard against non-physical or extra-supernatural threats which is where the suggestion is driven towards.