To my knowledge no magic is present in the novel, however it is in the film.
Is there any out of universe explanation for this change?
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There is an explanation! Don Bluth, director of the movie, offers this:
As for the amulet, "The amulet was a device, or symbol, to represent the internal power of Mrs. Brisby. It also was a gift from her husband, a sign of his love. The stone/amulet had no power, itself. It was only when Mrs. Brisby's strength was employed that it could become a force. In many ways, it was an extension of Mrs. Brisby... a visual extension of an internal (and harder to show in a film) power."
Nicodemus was changed to a wizard "to create more 'mystery' around both Nicodemus and the entire rat colony. Once again, making Mrs. Brisby have to search outwards, and inwardly for help."
So it seems as though Bluth believed magic was the easiest way to visually show the internal strength of Mrs. Brisby, much to the chagrin of book-lovers.