Cassidy Stay was at her home in Texas when her estranged uncle shot dead her parents and four siblings, also injuring her in the attack.
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After the 15-year-old recovered from her injuries, she made an inspiring speech at a memorial day for her family, quoting the famous books.
'Happiness can be found even in the darkest times, if one only remembers to tun on the light' she said.
An online campaign was then started to get the author to write to Stay, which eventually led to Rowling penning the note.
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In purple ink, and in the voice of Dumbledore, Rowling wrote the private letter and accompanied it with a wand, an acceptance letter to Hogwarts, a list of school supplies and a signed book.
Cassidy had previously quoted a line from the Prisoner of Azkaban, where Dumbledore addresses the students at the start of the film.
Since the tragic incident, more that $390,000 has been raised to help the teenager through a Go Fund Me site - which will go towards a trust fund for Cassidy, who will no doubt need a great deal of financial support without her immediate family still around.