Doctor Who and The Crown star Smith is set to play Prince Daemon Targaryen, a “peerless warrior” who is the younger brother of King Viserys and heir to the throne.
Cooke, known for Bates Motel, will portray Alicent Hightower, the daughter of Hand of the King Otto Hightower who was raised in the Red Keep, close to the king and his inner circle.
D’Arcy (Truth Seekers, Wanderlust) will star as Princess Rhaenyra Targaryen, the king’s first-born, pure Valyrian-blooded child who is a dragonrider.
Read more: Game of Thrones prequel gets production date
The three actors join Paddy Considine, who was previously announced as King Viserys Targaryen.
Many of the details about the new series remain under wraps, but it has been revealed that it will be set 300 years before the events of the original show.
House of the Dragon is one of four Game of Thrones spin-offs in the works at HBO. An untitled prequel starring Naomi Watts was not picked up by the network for a full series in October 2019.
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