The 48-year-old star, whose television credits include Doctor Who and Strangers, has landed a series regular role on the HBO programme, Deadline has reported.
Marquis Rodriguez, Richard McCabe, John Heffernan and Dixie Egerickx have also joined the cast.
They join previously announced stars including Naomi Watts, who headlines as a charismatic socialite hiding a dark secret, and Poldark’s Josh Whitehouse.
The Game Of Thrones prequel takes place thousands of years before the events of the main storyline, with HBO saying it “chronicles the world’s descent from the Age Of Heroes into its darkest hour”.
The eighth and final season of Game Of Thrones, which stars Emilia Clarke, Kit Harington and Peter Dinklage, is due to air next month.