How to complete Whence the Weeping in Phantom Blade Zero

Whence the Weeping in Phantom Blade Zero looks like a sidequest in the first Weeping Creek Valley area but is actually a major quest spanning much of the game, and quest tracking only unlocks later. Progress through Crimson Forest, speak to Jewel in the Village of Lu, then clear Pangzhen's main boss to get the Phantom Edge. Repair the Illusory Pipa, dispel the barrier northeast of Meigu Lake, and defeat the Mountain of Imprisonment region boss to finish.