A new Game of Thrones prequel series is in development at HBO. The show will be based on GoT author George RR Martin's Tales of Dunk and Egg novellas, whose events are set 90 years before A Song of Ice and Fire, books that were the source material for Game of Thrones.
The new series features the adventures of Ser Duncan the Tall (Dunk) and a young Aegon V Targaryen (Egg). Martin has published three novellas in the series so far -- The Hedge Knight in 1998, The Sworn Sword in 2003, and The Mystery Knight in 2010.
Variety reports that the project is in early development, and no writer is attached to it yet. But it is a 'high priority' title for HBO, as it looks to build on its post-Game of Thrones catalogue.