Once Promised 上古情歌 is a drama adaptation based on a novel by Tong Hua 桐华 that takes inspiration from a collection of scrolls called the Classic of Mountains and Seas. It got on my radar because of the names attached to the project such as Huang Xiaoming, Victoria Song (Ice Fantasy) and Sheng Yi Lun (Go Princess Go).
Unfortunately, Baron Chen backed out due to a broken leg and he will be replaced by Zhai Tian Lin instead.
In writing this post, I inadvertently read the ending thanks to too much information found in c-drama synopses but I'll spare you the details. Once Promised 曾许诺 is the prequel to Lost You Forever 长相思 set during a legendary era of Chinese lore about the epic romance between a warrior (Huang Xiaoming) and a maiden (Victoria Song).
For synopsis, cast and ratings, see 2016 best c-dramas.