The author posted on his blog on 15 April, sharing the news that actor Fabien Frankel would join the forthcoming series in a key role.
He will play the knight Ser Criston Cole, a member of King Viserys I Targaryen's Kingsguard, whose actions helped pave the way to civil war.
“He is the common-born son of the steward to the Lord of Blackhaven. He has no claim to lands or titles, all he has is his honour and his skill with sword and lance,” Martin wrote of Ser Criston in his blog.
“He is a challenger, a champion, cheered by the commons, beloved of the ladies. He is a lover (or is he?), a seducer (or is he?), a betrayer (or is he?), a breaker of hearts and a maker of kings.”
He continued: “Fire & Blood readers know him as Ser Criston Cole. House of the Dragon viewers will know him as Fabien Frankel.”
Frankel already has a small connection with Game of Thrones, as he appeared with original series star Emilia Clarke in the 2020 romantic comedy, Last Christmas.
He joins previously announced castmembers including Matt Smith, Paddy Considine, Olivia Cooke and Emma D’Arcy.
“Welcome to Westeros, Fabien,” Martin wrote. “And do keep that sword sharp.”
House of the Dragon follows the Targaryen family in Westeros. It takes place 300 years before the events in Game of Thrones.
Production is expected to begin in the UK this month, with a series release date planned for 2022.
This weekend will mark 10 years since the first episode of Game of Thrones aired on HBO.
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