Per EW, the actor will play "Hizdahr zo Loraq", a character who is introduced in George R.R. Martin’s fifth “A Song of Ice and Fire” novel A Dance with Dragons.
Hizdahr is described as the “young scion of an ancient Meereenese family who crosses paths with Daenerys Targaryen.”
Meereen the largest of the three great Slaver Cities of Slaver’s Bay. The others are Astapor (where Dany acquired her slave army) and Yunkai (which Dany’s army liberated in the season 3 finale).
Thrones returns to HBO next spring.