Simple intro on Zhong Kui:

Zhong Kui, we are told, first made his appearance during the reign of Kai Yuan (713-742 AD) when the Emperor fell ill with an incurable fever. One evening the Emperor dreamt of a demon with a "snout like a calf, one foot shod, and the other bare, the other shoe hung down from its body, and in its girdle it had stuck a large fan of bamboo paper." This spectre had stolen some personal items of the Emperor and was running with them through the palace halls. Just then a giant dressed in a hat and gown of deep blue emerged and chased the demon down, gouged out its eyes and ate the demon on the spot. When the Emperor inquired as to his identity the demon eater replied that he was Zhong Kui of Zhongnan Mountain who after failing to attain an honourable military post vowed that he would continue to serve the Emperor in the afterlife by exterminating all spectral evil. Hearing this the Emperor awoke and soon recovered his health. Recognizing this as a good omen, the Emperor ordered the painter Wu Daozi to render a portrait of the demon to be posted as a deterrent against demons.