Mern Gardener, also known as Mern IX, is a character mentioned in special features of the first season. He was the last of the Gardener Kings of the Reach. When Aegon the Conqueror invaded Westeros, King Mern joined forces with Loren Lannister, the King of the Rock, to face Aegon in battle.
At first the united armies of Lannister and Gardener seemed to be winning, pushing back Aegon's troops. However, once Aegon unleashed his three dragons, Vhagar, Meraxes and Balerion the Black Dread, even the combined might of the Kingdoms of the Reach and the Rock could not stand up to them. Four thousand men perished, consumed by the dragons' fire, earning the site of the battle the name of 'the Field of Fire.' Among the dead was King Mern himself. With Mern's death, the Gardener dynasty became extinct.
The Tyrells were one of the most powerful houses in the Reach, and Harlen Tyrell was steward to the Castle of Highgarden. When he surrendered to Aegon, Harlen Tyrell was named Lord Paramount of the Reach and Lord of Highgarden.