Halloween Nanka Dai Kirai

One Halloween night, on the outskirts of "Demon King's Hill," a town inhabited by magicians and other mysterious creatures. The main character, a young girl named Sentence, enjoys using magic to make what resembles message bottles. Today, on her way home from helping a girl in her class, she is sending out letters near the "Colorless Forest," believing that they would reach her missing childhood friend, "Sent". Then, she remembers rumors of a monster...?