Harren Hoare, better known as Harren the Black and Black Harren, is an unseen character in Game of Thrones. He's dead by the time of the series and has only been mentioned in dialogue.
Harren the Black was an ironborn King who ruled over the Iron Islands and the Riverlands from his castle, Harrenhal. Harrenhal was the biggest castle in the entire history of the Seven Kingdoms and was built to be invulnerable to any attack by land.
However, when Aegon Targaryen invaded the Seven Kingdoms, King Harren's vassals from the Riverlands, led by Edmyn Tully, revolted against him, while Aegon the Conqueror unleashed his dragon, Balerion the Black Dread against the massive stronghold. The King of the Iron Isles and the Riverlands and his sons perished as dragonflame consumed Harrenhal, leaving behind blackened and melted ruins.
In the books
In the A Song of Ice and Fire novels, Harren the Black was the last King of the Iron Islands and the Riverlands, grandson of Harwyn Hardhand, a King of the Iron Islands who took the Trident from Arrec, the Storm King. Harren the Black was a vain, bloody tyrant hated by his vassals.