The Other Heir of Magic is a character from the Land of Stories series, whose identity is yet to be revealed. He/she is mentioned only once in A Grimm Warning, ch 21, by the Fairy Godmother to Mother Goose.
In The Wishing Spell, Alex Bailey is mentioned as the "heir of magic". She starts her training as a fairy at the end of The Enchantress Returns, and succeeds the Fairy Godmother in A Grimm Warning. However, shortly before she dies, the Fairy Godmother asks Mother Goose to keep her secret - that there is another heir.
"If Alex hadn't proven herself to be the true heir of magic, I wouldn't be lying in this bed. [...] If she ever knew there was another - if she ever found out who they are - she would be fooled the same way I was and it would destroy her."
- ↑ TLOS III, ch 21, p. 344