Mistag is a cave goblin who plays a big part in The Lost Tribe quest and starts Death to the Dorgeshuun. The rag on his left shoulder signifies the arm he lost from a mining accident a few years ago. He will buy ores off of you (if you have mined them in the Dorgeshuun Mines). He buys iron ore for 13 coins and silver ore for 60 coins.
He can show you the way to Kazgar near the Lumbridge Castle cellar and to Dartog near the Lumbridge watermill dungeon after completing Death to the Dorgeshuun.