A Remembrall glows red when the owner has forgotten to do something.
”
From the Story
Harry reaching for the Remembrall
"A Remembrall is a glass ball the size of a large marble that reminds its owner if he or she has forgotten something. It is full of white smoke, which turns scarlet if the person who is holding it has forgotten something.
"Neville receives a Remembrall from his grandmother, which instantly glows red the first time he holds it; Neville can't, however, remember what he has forgotten.
"Harry catches Neville’s Remembrall in a spectacular dive after Draco Malfoy throws it across the school grounds. Professor McGonagall witnesses Harry’s catch, and instead of punishing him appoints Harry Gryffindor Seeker."
Location
The Remembrall can be purchased from Wiseacre's Wizarding Equipment down Diagon Alley for 14 Galleons. In Philosopher's Stone Chapter 9, Moment 1, it can be found in the third soom layer - Harry is reaching out to it and the player can snatch it from the air.
Use
The Remembrall has no current known use on Pottermore.
