Art for the Mid-Autumn festival
Yin Yu here refers to the moon goddess Chang'e. According to legend, Chang'e may descend from the moon to earth to her beloved husband hero Hou Yi only on the Mid-autumn holiday. I believe that parallels between the legend of Chang'e and Quanyin were intended.
The plot of the legend:
Hero Hou Yi shot down the extra suns in the sky with arrows and saved the people from drought, and the gods gave him an elixir that would make him a celestial. But Hou Yi was not interested in heaven, he wanted to live on earth with his beloved wife Chang'e. He left the elixir at home and went off to do heroic deeds. Then the envious Panman sneaked into the house to take the elixir by force and become a celestial himself. Chang'e drank the elixir herself to prevent trouble, and flew to the moon. On this day, fate divided them into different worlds and they missed each other. In culture, they are compared to the moon and the sun, which cannot meet.
Another version of the legend begins with the fact that Hou Yi and Chang'e were celestial beings, and the killed suns were the children of another god, but Hou Yi killed them anyway, solving the problem for mortal people radically. For this, both were banished from heaven. Chang'e scolded her husband for his guilty in making her mortal. And in order to help his wife Hou Yi started searching for elixir. If one person drinks it, he will become a god, and if two, they will gain immortality. The couple planned to drink it together, but Chang'e desired the second option — to become a goddess - and secretly drank it alone, flew to the moon and was doomed to eternal loneliness.
The endings vary from Hou Yi building a palace on the sun to Hou Yi dieing of grief.
There are many variants of the legend, but the general outline is the same — there was an artifact intended for Hou Yi (a hero related to the sun), but Chang'e (related to the moon) suffered from it, either from her vanity to become a goddess, or "taking the damage on herself". In some stories she doesn't let a villain use an artifact in evil way. Then Hou Yi returns home and doesn't understand where is wife.
On the same day when Chang'e became a goddess of moon, Jian Yu, against the rules, took the Brocade Immortal and gave it to Yizhen. Yin Yu dashed to take it off Yizhen, got punished for all the murders, fell from heaven and became a significant figure in the opposite "night" demonic world under the name Xiàxián Yuè Shǐ — the Waning Moon Officer. And Quan Yizhen wasn't consiousness when all this happened, dissapearance of shixiong was a surprise for him. He missed and was searching for his shixiong in the mortal realm.