In this film, the magic mirror has been reduced to a simple hand mirror. It was given to Evie as a tool to obtain Fairy Godmother's wand. Like most previous versions, it can answer and question, but this time by showing the answer on the glass, and it must be asked a question in rhyme. It will only respond for the Evil Queen or Evie. Some questions posed it are "Where does Fairy Godmother's wand stand?" and "Who is the biggest jerk in the land?"
The hand mirror that contains a shard of the Magic Mirror starts the short animated episodes off when Evie asks it to show her the future. To see "what happens when" their "stories unfold".
Evie doesn't use the mirror or it's magic like she did in the film, both she and Mal are trying to be good by not using the mirror or Mal's Spell Book. But Evie did talk to it in Voodoo? You Do, after looking at herself in larger a mirror in a shop on the Isle of the Lost. She wanted to make sure that both the mirror and the reflection it gives of her know that she stills loves them: and so the mirror itself wouldn't get jealous. However Evie does use it's magic in Mal-lone, as she asked it to show her where Zevon is.
The spirit of the mirror's face/head can also be seen as a large mask in the Facilier shop as Mal, Evie, Ben and Audrey walk into it.
The magic mirror was mentioned in Descendants 2 when Evie was trying to convince Mal to donate her spellbook to the museum.
Isle of the Lost: A Descendants Novel
In the book, there is a magic mirror standing in a room at the forbidden fortress on Doom Island, however: it is unlikely this is the magic mirror, since that mirror is broken and this one is whole. Also, it would be unlikely for the Evil Queen to have the magic mirror on Doom Island since she lived in Charmington. And the Evil Queen is last seen with it there and dies shortly after.
CJ Hook tries to use it to locate something. She ended up learning that it only works for Evie and her mother.