Holiday - Batman: Arkham Knight: A Flip of a Coin

Dubbed by the press as Holiday, this serial killer targetted members of Gotham's organised crime families on key public holidays, killing them with silenced .22 pistols, before leaving behind iconography synominous with festive occassion.

 

These modified guns were all untracable, thanks to the murderer filing off the serial number, making it impossible to deduce their true identity from them alone. 

One of Batman's earlier cases, Alfred Pennyworth would remember the affair to be a partcilalrly ghastly and memorable one. Futhermore, due to the case's significance to Harvey Dent, he and Two-Face would keep old evidence from the case, putting them on display at Hell's Gate Disposal Services.