The story is a biographical piece about singing sorceress Celestina Warbeck. The character is mentioned in the Harry Potter series of books as being Molly Weasley's favorite singer.
"Celestina Warbeck is one of my favorite 'off-stage' characters in the Harry Potter series," Rowling writes. "Although we never lay eyes on Celestina during the whole seven volumes of the Potter books, I always imagined her to resemble Shirley Bassey in both looks and style. I stole her first name from a friend with whom I worked ... 'Celestina' was simply begging to be scooped up and attached to a glamorous witch."
The story is available to those with a free account on the Pottermore website, and is also posted on Today.com. Celestina is described as a former Gryffindor with a larch and phoenix feather wand, who has the "ability to drown out a chorus of banshees."