Lord Eddin Mercer is a recurring character in the fourth, fifth, and sixth seasons. He is the head of House Mercer and is the lord of the castle of Dunharrow, and a powerful vassal of House Blackgard, having been of Trevyr Blackgard's closest friends since childhood. He also spends a brief period of time as the personal steward of King Tommen Baratheon. He is the husband of Lady Myrianne Mercer and the father of Roman, Grayson, and Celesse Mercer.
During his time with the Sons of Asterion, Eddin was known for his exceptional archery skills, and was therefore known as "The Black Arrow".
Eddin was born during the Fourth Border Wars as the thirdborn child of Lord Torrhen and Lady Katarina Mercer. Not long after he was born, Lord Trovar Blackgard (who his bastard uncle Gordin Rock was squire for) was murdered by House Keller.
Though he doesn't physically appear, he and his wife attended Elyse Blackgard and Marlen Rosby's wedding (according to Codin Bruckstone, the steward of Ebonheart), and Trevyr later mentions that he was able to catch up a little bit with him at the wedding feast.
Lord Eddin and his wife, Lady Myrianne Mercer, are invited to attend the royal wedding of King Joffrey Baratheon and Margaery Tyrell. In a brief conversation he has with Cersei Lannister, he mentions that he has appointed his secondborn son Grayson Mercer to rule Dunharrow in his stead while he is away, and