It's loosely based on a Tang Dynasty story
I added this info to a wikipedia article before I even learned about The Promise. Once I read about the movie, I knew that they adapted it from this story.
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Kunlun Nu ("The K’un-lun Slave") was a romance written by P’ei Hsing (c. 880) during the Tang Dynasty. It takes place during the Ta-li reign era (766-80) of Emperor Daizong and follows the tale of a young man named Ts’ui who enlists the aide of Mo-lê, his negrito* slave, to help free his beloved who was forced to join the harem of a court official. At midnight, Mo-lê kills the guard dogs around the compound and carries Ts’ui on his back while easily jumping to the tops of walls and bounding from roof to roof. With the lovers reunited, Mo-lê leaps over ten tall walls with both of them on his back. Ts’ui and his beloved are able to live happily together in peace because the official believes she was kidnapped by Chinese knights-errant and did not want to make trouble for himself by pursuing them. However, two years later, one of the official’s attendants sees the girl in the city and reports this. The official arrests Ts’ui and, once he hears the entire story, sends men to capture the negrito slave. But Mo-lê escapes with his dagger (apparently his only possession) and flies over the city walls to escape apprehension. He is seen over ten years later selling medicine in the city, not having aged a single day.
Mo-lê’s gravity defying abilities and agelessness suggests the fictional character might have been a practitioner of esoteric life-prolonging exercises akin to Chinese immortals. According to a tale attributed to the Taoist adept Ge Hong, some hunters in the Zhongnan Mountains saw a naked man whose body was covered in black hair. Whenever they tried to capture him he “leapt over gullies and valleys as if in flight, and so could not be overtaken." After finally ambushing the man, the hunters learned it was in fact a 200 plus year old woman who had learned the arts of immortality from an old man in the forest. Still, it was popular in folktales for immortals to sell medicine in the city, just like Mo-lê did. The hagiography of the immortal Hu Gong (Sire Gourd) says he sold medicine in the market place during the day and slept in a gourd hanging in his stall at night.
(http://en.wikipedia.org/wiki/Negrito#Literature)
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(The following was not added in the article)
*Negrito - can be any number of dark-skinned people hailing from Africa or Southeast Asia. The Chinese word for negrito is “Kun-lun”. During the Tang Dynasty, Arab merchants brought black slaves with them to China. These slaves were treated with respect by the Chinese and were even taken to have supernatural powers. Several stories from the Tang and Song dynasties involve these Kun-Lun slaves and their supernatural adventures.
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I saw a 20 minute clip and the only thing I liked about it was when the General started wielding his meteor hammer. The CGI was HORRIBLE! No amount of beautiful scenery or costumes will save a movie that heavily relies on special effects created by a 5th grader in middle school.