Made up story
Lan Xiying, a cancer girl, died after being tortured by the disease for a year, and her soul was wandering around.
On the other side of the world, after a young man heard the story of Nang Yingxue, he caught a thousand fireflies and put them in a plastic bag. The girl's soul passed by, and the fireflies that were almost suffocating in the plastic bag gave her their resentment towards humans and their own light, allowing him to be reborn as a monster and continue to live in the world. The condition is: avenge them - harm others