Ser Alliser Thorne became acting Lord Commander of the Night’s Watch after the murder of Lord Commander Mormont. Prior to this, he was master-at-arms, responsible for training the brother’s of the Night’s Watch. Before joining the Night’s Watch, Ser Alliser was a knight of House Thorne, a house that sided with the Targaryens during Robert’s Rebellion and fought to defend King’s Landing against the rebels. After the Mad King was defeated and King’s Landing fell to the rebels, Lord Tywin gave Ser Alliser two options: death or to join the Night’s Watch. He chose the latter and has been a bitter and vengeful man ever since.