Forget the War of the Five Kings: The running for whose Game of Thrones spinoff idea will see the light of day is turning out to be just as intriguing.

In a new Hollywood Reporter post, series writer and co-executive producer Bryan Cogman says his concept for a Thrones spinoff “is not happening and will not happen. HBO decided to go a different way.”

Cogman was one of five people who’d been working to develop various successor series set in author George R.R. Martin’s A Song of Ice and Fire universe. In June, the network greenlit a pilot based on the prequel idea put forth by Jane Goldman (X-Men: First Class). She will serve as showrunner, and she and Martin will executive-produce, alongside Vince Gerardis (Game of Thrones) and Daniel Zelman (Bloodline).

Per that potential series’ official logline, the action will take place ages before the exploits of Daenerys, Jon & Co. and will chronicle “the world’s descent from the golden Age of Heroes into its darkest hour. And only one thing is for sure: from the horrifying secrets of Westeros’ history to the true origin of the white walkers, the mysteries of the East to the Starks of legend… it’s not the story we think we know.”

In September, Cogman signed an exclusive deal with Amazon; he tells THR the death of his Thrones-related series means a “goodbye” to the world of the hugely popular fantasy drama.

“I am done with Westeros, he adds. “I’ll certainly miss it, but I’m excited to go out on my own and try to be in the captain’s chair of my own projects, armed with everything I’ve learned.”

Cogman started as an assistant to Thrones creators David Benioff and Dan Weiss and eventually became part of the series’ writing team and a co-executive producer. He is responsible for several Thrones episodes, including Season 3’s “Kissed by Fire,” Season 6’s “The Broken Man” and Season 7’s “Stormborn,” as well as “A Knight of the Seven Kingdoms,” which aired Sunday as part of the series’ final season.