No formal grammar rule I can think of would prevent use of the present tense in this narrative. Thinking of grammar as how we order and combine words to produce meaning, then this sentence makes perfect sense either way. The question is, how is your narrative structured? How is you narrator speaking?

If they are telling the story in the present while using past tense to describe past actions, then present tense would be a good choice for "But necromancy is forbidden."

However, if the narrator is telling the story completely in the past tense, then you'd want to stick to past tense in the sentence. If the narrator suddenly switches to present tense when they haven't been using it, then yes ... that would feel jarring to me.

If the narrator is more embodied and more a part of the story, that's one reason why they may sometimes speak in present tense. In that case, their choice of tense in this sentence would depend on what they mean. If they mean necromancy used to be forbidden but isn't now, they might probably say "was." If they mean necromancy is still forbidden, they'd say "is."

This isn't about grammar. It's about how carefully you are constructing your narrator's voice and what your narrator is trying to communicate.