Like ghosts, ghast are also the unquiet undead. But unlike ghosts, ghasts are only just part of ghosts that have already moved on, and repeat a tragic thing they did or the way they died. Ghasts are harmless unless and they only are a true threat if someone is is afraid. Ghasts can not move on since they are only part of the ghost that moved on. And they will remain earthbound till a hundred years or so. The Spook says that they cannot move on by giving them a good talking, but they can still move on, which was proven in the first book when Mam went up to Hangman's Hill and the moans grew less to help poor Tom finally sleep. However, it is still unknown how she did this, and we may never know due to her death.