This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Soul Cartel
Airen's mother died in the magical process of splitting her life in order to bring a new witch into existence, but she is later taken in by the witch in charge of the grand library, Inea. Sadly, another witch named Liselot betrayed the witches in the hopes of forming an alliance with the demons, and attempted to force Inea to hand over a powerful magic book. Inea thwarted their efforts by sealing the entire library and throwing Airen into a separate room. Since it was a new spell with no known means of dispelling it took three months to free them. Liselot survived by magically creating supplies for herself. Airen could not since she only knew the basics of magic. Despite Liselot and the demons torturing her in the hopes of forcing her to free them, Inea created food for Airen till her last breath. In the end, the demons died due to lack of souls, Liselot was executed for her treason, and Airen was the only one who left the incident alive.