Dragon King

Dragon King (応竜), is the fourth and final Guardian Beast set to prevent the assimilation of the world by sacrificing two-thirds of the population of the world.

Appearance
The appearance of the Dragon King is unknown except that it is a dragon.

Abilities
The abilities of the Dragon King are unknown. However, the dragon emperor shards related to the Dragon King provide knowledge to conduct the level-100 limit break ceremony, to bless people to allow them to use Way of the Dragon Vein magic, and to use Dragon Sanctuary.

Background
As a Guardian Beast, the Dragon King is a backup safety feature to fight against the Waves of Calamity. It was meant to act as a last act of defense in case the Legendary Heroes were not powerful enough or willing to defend against the waves. However, their help comes with a great cost as the only way they can stave off the waves is by killing the residents of their world to power a barrier which repels the waves. In total, 2/3rds of the world's population would have to be killed in order to permanently repel the waves. Unlike the other Guardian Beasts however, the Dragon King is not unsealed by the previous Guardian Beast being defeated. The Dragon King is unsealed through knowledge gained by gathering the dragon emperor fragments.

Volume 16
After defeating Leludia of Takt's harem and consuming her Dragon Emperor fragments, Gaelion gained the knowledge to unleash the Dragon Emperor. However, since this would require Gaelion to sacrifice himself, Naofumi declined Gaelion's offer to do so and asked that Gaelion would keep this knowledge secret.

Trivia

 * Ōryū is the Japanese reading of Yinglong (应龙/應龍), a winged dragon and rain deity in Chinese Mythology, which is one of the Four Benevolent Animals.
 * Dragons collecting dragon emperor shards to unleash the Dragon King parallels Qin Shi Huang, the first emperor of Qin (or China) who conquered and united the country. The emperor was also called the Dragon King.