The play will be written by Duncan MacMillan and directed by Dominic Cooke. MacMillan's script will be based on a story outline provided by George R.R. Martin.
The project is notable as the first Game of Thrones-related project featuring a new story that will draw on established characters. Ned Stark, Tywin Lannister, Jaime Lannister and Barristan Selmy play major roles in this story, as do the briefly-appearing Lyanna Stark and Rhaegar Targaryen. However, these roles will be played by new (and, given the medium, regularly changing) actors.
The project is also interesting because it may indicate a change of direction from George R.R. Martin, who had previously shot down all suggestions for a spin-off or prequel about the Great Harrenhal Tourney or the succeeding Robert's Rebellion. Clearly he's now - at least partially - changed his mind on that, making a spin-off TV show or film about the Rebellion itself potentially more likely.
The stage play is expected to arrive in Broadway (New York City), the West End (London) and Australia in 2023.