The Bandon Banshee was a banshee who was supposedly defeated by Gilderoy Lockhart. It was actually defeated by a witch with a hairy chin. The book Break with a Banshee is most likely centred around this Banshee's defeat. This achievement seems to have been one of Lockhart's personal favourites, as he makes two explicit references to it, and when he visits Hagrid, he supposedly "bangs on about some banshee he banished," likely a reference to this supposed achievement.