Tarnish the Crown is a quest available in The Elder Scrolls Online. King Demog and his goblins killed Gugnir's brothers and left him for dead. He asked the Vestige to avenge his brothers by killing the goblin king.
North east of Port Hunding, the Vestige will meet a man who seems to be injured. He tells them that the goblins are going to kill them all. His brothers and five guards are dead and he thinks he hasn't got long to live. He asks for assistance to kill the goblin king and take his crown to prove that he is dead. The informs them that they are planning to attack Port Hunding. He will gather his streng andth go back to town once they are finished speaking to him.
Follow the marker to the goblin camp, and try to get near the king. The camp is full of goblins, and at the other end of the camp they will find the king. Defeat him and loot his body for the crown.
Head back to town to meet up with Gugnir. Give him the crown and tell him the king is dead. He is very pleased as he's been expecting an attack any minute. The town is safe and he rewards the Vestige with a pair of Goblinslayer Chausses and some .