We have been adding mild magic to some of our historical games, we call it “legendary magic” (or “medieval-legendary” instead of medieval-fantasy) especially in so-called “Arthurian” 5th century context. It’s a few (not powerful) spells selected from another, larger list that we use in fantasy games.
Most often it’s a blessing (add +1 to the die roll during one game turn in the next fight) or curse (-1 to the die roll during one game turn in the next fight) which is cast, prayed, or sung, by a monk, priest, druid or bard. It needs that the recipient understands the language and can believe it will be effective (we don’t say clearly it’s a real magic effect or only a shared belief in the effects of the prayer or song). We also have a few prayers or spells which briefly change the weather (for fog, or rain, or sun) …or rather, it happens that the weather “naturally” changes for a few minutes after these prayers or spells are said.
We have sometimes used the same “spells” in other historical contexts; for exemple in c.1700 pirate games for use by a vodoo priestess leading marooned slaves; and once in a TYW game where one player character had to search a village for a witch while all other players were engaged in a larger battle all around. In all cases we always say that it’s unclear if what happened was real magic or if the player characters misunderstood a natural event by superstition.