Creative Prompt:

Your protagonist, a renowned but jaded detective, receives a mysterious letter that hints at an unsolvable case from the past. The sender - an anonymous and seemingly omniscient entity. The detective becomes obsessed with this enigma, their professional instincts clashing with personal demons. Write a noir-style narrative exploring the detective's evolving relationship with this faceless antagonist, their descent into obsession, and how they grapple with their past.