Eight rising stars have been cast in main roles, set to star alongside Naomi Watts, who was previously announced as a series lead.
What roles each new actor is playing is being kept under wraps, but a little more about the plot has also been teased.
"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," HBO says in a logline.
Meanwhile, SJ Clarkson has joined the prequel as pilot director and executive producer. She's directed episodes of Jessica Jones, Orange is the New Black and Succession.
Josh Whitehouse has also been previously announced as a major cast member.