Thistle

Thistle is the mage responsible for the creation of the dungeon on The Island and a jester who served in the court of King Delgal during the Golden Age. His actions make him the primary antagonist of the manga Delicious in Dungeon.

As the lord of the dungeon, the Lunatic Magician has a great amount of power over both the structure of the dungeon and the monsters that inhabit it. His lifespan has also been extended by the dungeon's power. He is incredibly loyal and protective of the king, and is willing to destroy anything he perceives as a threat to his throne or kingdom. The elf was present in the king's life since his birth and for major events such as his wedding and coronation.
During King Delgal's reign, he suggested that Thistle study magic, which is where the elf flourished. However, among the elven and dwarven relics that Thistle studied was something of the dark arts. Thistle became obsessed with power and began his descent into madness, culminating in the elf casting a curse of immortality over the country and creating the dungeon. The Lunatic Magician also imprisoned the guardian of the Golden Country, the Winged Lion, in the depths of the dungeon. (Source: Dungeon Meshi Wiki)