The project will be written and created by "Game of Thrones" co-executive producer Bryan Cogman.
Game of Thrones Season 8 premieres sometime in 2018 or 2019.
There's a fifth Game of Thrones prequel series in the works, and the project's writer is a familiar name to the fantasy drama's fans with a massive amount of Westeros experience. As with the other prequels, David Benioff and Dan Weiss will not be involved.
The other four writers announced by HBO earlier that month include Max Bortstein (Kong: Skull Island), Jane Goldman (Kingsman: The Golden Circle), Brian Helgeland (A Knight's Tale, Robin Hood) and Carly Wray (Mad Men, The Leftovers).
Entertainment Weekly, who broke the news about Cogman's involvement with this fifth prequel, says he "developed the project by working closely" with Martin, and it remains unclear if Martin himself will actually be receiving co-writing credit on any of these projects that get the greenlight or if he'll be involved in a less demanding capacity.
Is there now a favorite Game of Thrones prequel to eventually find its way on the air?
As previously reported in May, HBO and George R.R. Martin, whose A Song of Ice and Fire novels serve as the source material for the expanding GoT franchise, are working with five different writers on five different prequels. Cogman was brought on to Game of Thrones in Season 1 as an assistant to Benioff and Weiss, and he's written several episodes of the series since then.
Martin went on to give more details about Cogman's participation, revealing that the series he will be working on is an adaptation and is set during a "very exciting" era of Westeros. Those who have kept up with word of the prequels will know that Martin has already ruled out tackling Robert's Rebellion or anything to do with his ongoing Dunk & Egg saga, but that doesn't mean the dragonglass mine is empty just yet.
Although fans shouldn't expect to see all five shows come to air, George did reveal two or three could make it to the pilot stage.
Given the material that Martin has produced set in Westeros, Cogman's series nearly certainly concerns Targaryen history.