Foreword: Thousands of years ago, Bai Zhan and Bai Zhan missed reincarnation because they cared about each other.
Ksitigarbha Bodhisattva was moved by the affection between the two, and Wenquxing persuaded them, so he allowed them to stay in the human world and be reincarnated.
But before reincarnation, the two need to lead 881 wandering souls to the underworld.
Bai Zhan and the two agreed, so they stayed in the world and led the wandering souls back to where they should go - the underworld.