He looks like he's in charge of the miners.
Mistag is a cave goblin who plays a role in the quests the Lost Tribe, Death to the Dorgeshuun, and the Chosen Commander. He also plays a small part in Land of the Goblins. The reason for the rag on his left shoulder is that he lost his arm in a mining accident.
He also buys unnoted ores off you (they do not have to be mined). 60 coins for each
silver ore and about 13.6 coins (rounded down) for each iron ore (13 for 1, 27 for 2, 40 for 3, 54 for 4, 68 for 5, 81 for 6, 95 for 7, 108 for 8, 122 for 9, 136 for 10, 272 for 20, 367 for 27).
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.
He is the owner of the
brooch found during The Lost Tribe. It can be returned to him after the quest.
When the Goblin Bow emote is performed in front of him he repeats the bow himself
When the Goblin Salute is performed in front of him he says "Eeek! The war-dance!"
The word misstag is the Swedish word for mistake. Since Mistag lost one of his arms in an accident, his name is likely a reference to misstag.