Origin: Japanese Mythology
Dragons Slain: Nameless dragon hidden by a black cloud
Background: A long time ago, the Emperor became ill and could not sleep at night.  He complained of disturbances and a great sense of oppression from sunset to sunrise.  His courtiers stayed up through the night to discover the cause. Some kept vigil in and around the Imperial chamber; meanwhile, others on the opened roofs and galleries in the courtyard of the Palace.  None of them expected to see the strange nocturnal visitor that approached. 
As soon as the sunset, a black cloud came from the eastern horizon of the capital,  and landed on the Purple Hall of the North Star, which is the roof under which the Emperor slept.  Those that kept watch in the Imperial chamber heard the unusual sounds, like an animal with huge claws scratching, came from the cloud. They also saw the Emperor became immediately afflicted with a nightmare. This lasted until sun up, when the black cloud withdrew. 
It became very clear that, should whatever monstrosity hidden by the black cloud not be killed soon, the Emperor would soon die. He grew sicker and thinner with every passing day, but the palace sentinels were terrified of the creature.  The guards were too afraid to fight the creature hand-to-hand, and none of the local archers could hit the creature in the dark, though the entirety of the Emperor's own guard of archers tired again and again. 
The ministers sent out an Imperial messenger to the samurai Yorimasa. 
Yorimasa, the Dragon Slayer
As a man of courage and resource, Yorimasa did not waste a movement in preparation.  He strung his best bow and took steel-headed arrows.  He chose his favorite retainer, a soldier he marked as the bravest and strongest in his company, to help him in this task.  He put on his armor, and they set out. 
Yorimasa did not lose heart at the alarm and gloom of the palace and the members of the court.  He and his retainer waited patiently for the night.
Yorimasa waited in concealment outside the palace.  As soon as the sun went down, the night became stormy. Hurricane winds and lighting flashes were underscored by roaring thunder.  Some say the wind shrieked like a pack of wild demons,  but nothing daunted Yorimasa. 
It wasn't until midnight that he saw the dark cloud approach, and as soon as he saw it, he bade his retainer be ready with a blazing torch and sword.  Yorimasa followed the clouds movements till it landed on the roof, and then he stared at it, waiting.  He saw the form of a large animal obscured by the cloud when lightning flashed.  He kept his focus on where he saw the head, so when the darkness swallowed up the animal again, he caught the glare off the creature's right eye, then the glare from the left. 
With that, Yorisama fitted the steel arrow into his bow, and sent it flying into the creature's left eye. A howl, a thud, and the sound of something writhing on the ground told Yorisama that his aim was true.  Yorimasa and his retainer rushed upon the monster; with swords they stabbed the now-visible animal nine times.  When it was dead, they called for the court to see it.
The dragon was as large as a horse and had the head of an ape, body and claws of a tiger, tail of a serpent, wings of a bird, and the scales of a dragon.  The Emperor gave orders that the skin of the monster should be kept as a trophy and curiosity.  As predicted, the death of the creature brought about a drastic improvement of the Emperor's health. 
As a reward, Yorimasa was presented with Shishiwo, a sword with a name that means 'the King of Lions.'  He was promoted at Court, and he married the Lady Ayame, the most beautiful of ladies-in-waiting in the Imperial Court. 
- Yorimasa was an archer of great skill. 
- The dragon plagued the Emperor by haunting him and giving him grave nightmares, depriving him of sleep. 
- Covered by the darkness of the night and the darkness of the cloud, no archer could hit the dragon. 
- Yorimasa took great patience to spot the dragon and hit it in the eye with a steel-headed arrow. 
- He and his retainer then stabbed the dragon to ensure its demise. 
For more information on footnotes and references, please see the bibliography.