Cruel Dragon

Cruel Dragon (残酷竜, Zankoku Ryū) was a very ancient and powerful dragon that once laid waste to civilization before being weakened and nine seal stones for more than eight hundred years. It was killed by Saitama.

Appearance
Cruel Dragon is a large, draconic monster with massive spikes protruding from its head and tail. It has three pairs of giant wings, six arms, several rows of sharp fangs and three pupils on each eye.

Personality
Cruel Dragon is incredibly cold-blooded and sadistic, which is what lead to its namesake. Centuries of imprisonment increased its bloodlust exponentially and made it determined to wipe out humanity.

Monster Association Arc
During the fight between Saitama and Garou, as a result of a powerful attack from Garou that even bulges the Earth, a temple with the head of Cruel Dragon is risen from under the sea.

Supreme Hero Arc
After defeating the Ninja Village Leader, Saitama accidentally gets in the middle of a battle between the Saints, a group of warriors dedicated to keeping Cruel Dragon sealed and Deathbone, a cult of "dragonborn" trying to release the monster by collecting all nine stones. After being explained the situation, Saitama decides to unseal Cruel Dragon so that he can defeat it and the Saints no longer have to worry about keeping him captured. Once Cruel Dragon is unleashed, it announces its intentions of destroying humanity and tries attacking Saitama to no avail. Cruel Dragon is then nonchalantly killed by Saitama with a single punch.

Abilities and Powers
Cruel Dragon's true power is hard to gauge due to its lack of a disaster level and how quickly Saitama killed it. It was definitely a highly dangerous monster, with its fire breath being able to scorch large areas of land in just a few days. Despite its great power, a large group of unknown soldiers managed to weaken it, though it cost them many casualties. The Saints were worried that Cruel Dragon would destroy all of humanity if it was unsealed.

Fire Manipulation: Cruel Dragon could breathe fire from its mouth.

Flight: Though unseen, the Cruel Dragon had 3 sets of wings with which it could likely fly.

Trivia

 * Cruel Dragon is a reference to the storytelling trope of an ancient evil that has been sealed away and threatens to return, which is used in many classic stories such as the myth of Pandora's Box. This trope is subverted by having Saitama wake the evil up himself to just kill it and get it over with.