[House of the Dragon] is based on an earlier period in history called the Anarchy."
Once he died, her claim to the throne was challenged by her cousin and civil war broke out.
Sounds pretty similar to the synopsis ofHouse of the Dragon.

Milly Alcock’s Princess Rhaenyra Targaryen stands before the Iron Throne in ‘House of the Dragon’.Ollie Upton/HBO
House of the Dragonpremieres on HBO this Aug. 21.