House Tyrell of Brightwater Keep is a noble house from the Reach and a new cadet branch of House Tyrell of Highgarden. The house was created by King Joffrey I Baratheon, who stripped House Florent of Brightwater Keep and bestowed it on Ser Garlan Tyrell.
House Tyrell of Brightwater Keep was created after the Battle of the Blackwater, where Mace Tyrell sided with Joffrey against Stannis Baratheon. Mace's bannermen, the Florents, led by Alester Florent, supported Stannis. Alester was stripped of Brightwater Keep, and the castle was granted to Mace's second son.
Garlan Tyrell leads half the Tyrell army to take Brightwater Keep, currently held by Ser Colin Florent. However, he hears of Euron III Greyjoy's invasion of the Shield Islands, and marches to the Shield Islands instead.