Bandon Banshee

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 centered around this banshee's defeat.

This achievement seems to have been one of Lockhart's personal favorites, as he makes two explicit references to it, and when he visits Rubeus Hagrid, he supposedly "bangs on about some banshee he banished," likely a reference to this supposed achievement.