The Goblin King is an original character from The Land of Stories series. His only appearance is in The Wishing Spell, ch 11. He is co-ruler of the Troll and Goblin Territory together with the Troll King. They share a single crown. He is killed when "rocks fall" on the territory. 
Appearance and Personality
The Goblin King is not specifically described, though the trolls and goblins in general are said to be "grotesque and dreadful", with warts, sharp teeth and horrible hygiene. They keep captured humans as slaves.
"They were the type of monsters Alex and Conner used to have nightmares about when they were younger."