King 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. Their armies met at the Field of Fire, during which Mern himself was killed, and his kingdom fell to Aegon.
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 were King Mern himself and all of his sons; thus the Gardener dynasty became extinct. Mern's steward, Harlen Tyrell, surrendered Highgarden to Aegon and was named Lord Paramount of the Reach, Lord of Highgarden and Warden of the South.
Seem that King Mern wasn't inclined to a war initially, like it would have been a sure defeat, but is implied that Lord Harlen Tyrell had suggest Mern to going to war to not appear weak and coward, with the intention to reclaim the Reach for himself.
In the books
In the A Song of Ice and Fire novels, Mern's story is the same.
Kings of the independent Seven Kingdoms
|Kings in the North|
|Kings of the Rock|
|Kings of the Iron Islands|
|Kings of the Reach|
| Kings of the |
Mountain and the Vale
|Princes of Dorne|
|Kings of the Rivers and Hills|