Sinner's End is the fourth district investigated in Criminal Case: Mysteries of the Past.
A district known for being a nest of alcoholism, prostitution, gambling, and other illicit activities enacted by the Irish mafia of Concordia, Sinner's End focuses on two ongoing plots at the same time for its storyline: first there's the Flying Squad's efforts of bringing down Mad Dog and The Viper, the two leaders of the aforementioned Irish mafia controlling the entire district, by trying to find enough dirt on them and perform and arrest to put them away for good. However, things soon prove to be even more difficult when the presence of a Serial Killer known as the Scarlet Slayer makes itself known to the public, targeting the prostitutes of Sinner's End and providing another challenge for the Squad to overcome by uncovering their identity and stop their bloody killing spree.
- Sinners and Saints: A doctor is beaten to death them dumped on the streets behind a pub.
- Eyes Wide Shut: A pimp is stabbed through the eyes with a candlestick holder inside of a brothel.
- Blue Blazes: An Irish moonshiner is doused in alcohol then set ablaze near his delivery truck.
- Overkill: An Italian gangster is stabbed in the back with a poisoned blade in a park.
- Death Is a Cabaret: A prostitute has her stomach sliced open inside of a cabaret.
- Slayer's End: An escaped convict is strangled to death inside of an opium den.
Tropes:
- Arc Villain: A Big Bad Duumvirate between the Flanagan siblings, Finley "Mad Dog" Flanagan and Fiona "The Viper" Flanagan, who serve as the leaders of the Irish mafia controlling the entire district with an iron fist. In addition, Fiona is also revealed to be the Scarlet Slayer, a Serial Killer murdering the prostitutes of Sinner's End and terrorizing the district, making it a strange case of a character serving as the main villain of two ongoing plots at the same time.
- The City Narrows: Sinner's End serves as the seedy portion of Concordia, being a district bursting with crime, prostitution, lots of alcohol, underground fight clubs, etc, all of it controlled by the Flanagan siblings, Mad Dog and The Viper.
- The Irish Mob: All of Sinner's End is in some way controlled by the Irish mafia of Concordia, from the alcohol production and pubs to the prostitution business, with Finley Flanagan and his sister Fiona at the helm of the whole operation. One of the main focuses of this district is finding a way to ease the stronghold the Irish mafia has over Sinner's End by arresting their leaders.
- Recurring Character: The main recurring characters for Sinner's End are Finley Flanagan, Fiona Flanagan, Katherine Woolf, Marla de Paradis, Madam Xiang, Adrienna Brassiere, Gladys Perrin, and to a lesser extent Seamus O'Neill.
- Red Light District: Combined with The City Narrows mentioned above, the district of Sinner's End is basically just a giant red light district for Concordia.
- Serial Killer: Half of the main plot of Sinner's End is uncovering the identity of and capturing the Scarlet Slayer, a serial killer who for the past months has been murdering prostitutes all over the district for no apparent reason.
- Shout-Out: The aforementioned Scarlet Slayer, a Serial Killer and one of the Arc Villains of the district who has been murdering multiple prostitutes (including one named Annie), is a clear parody of Jack the Ripper.
- Two Lines, No Waiting: Sinner's End focuses on both trying to dismantle the Irish mafia's illegal operations on the district while also investigating the true identity of the Scarlet Slayer and put a stop to their killing spree. In the end, the two plots end up being Connected All Along in a way, as the Scarlet Slayer turns out to be one of the Irish mafia's leaders and is promptly killed by the other one (who in return gets arrested), giving a conclusion to both storylines at the same time.
