Summary
- A Dark Knight fan theory suggests that the Joker could be a former League of Shadows operative who uses their training to wreak havoc on Gotham as an individual agent of chaos.
- Batman Begins may have showed the Joker's origin story onscreen when Batman beats a group of League of Shadows henchmen, possibly leaving one of them with face scars with his arm blades and exposing him to Scarecrow's fear gas.
- Apart from the Joker's tactical skills, his wish to destabilize Gotham aligns with the League of Shadows' philosophy.
A clever DC fan theory proposes a new take on the Joker's origin story prior to The Dark Knight, tying it to Batman and the League of Shadows. Heath Ledger changed comic book movie history with his Joker performance in one of Christopher Nolan's most acclaimed movies, The Dark Knight, as he delivered the most intelligent, cruel, and authentic take on the iconic DC villain. Ledger's Joker is an even more mysterious figure by the end of The Dark Knight than he is at the beginning, and his nihilistic approach to villainy elevates him over other Batman enemies with clearer motivations.
Apart from building mystery around the character, the Joker's unknown origin story also gives the audience the option to come up with their own creative theories. From a military background to an Arkham backstory, every hypothetical Dark Knight Joker origin theory adds a new element to the villain's legend. After all, the Joker's wide array of skills doesn't seem to come from a regular citizen, but from a terrorist with a specialized background. Reddit user r/DickieIam goes a step further and not only explains the Joker's origin but also connects him to Batman and all of the Dark Knight's first enemies from Batman Begins.
Your browser does not support the video tag. Joker Could Be A Former League Of Shadows Operative 
In Batman Begins, Ra's Al Ghul enacts his plan to poison Gotham's water supply in Batman Begins with the help of a small team of League of Shadows acolytes. Batman fights Ra's Al Ghul's henchmen before thwarting the villain's plans and causing his death. As the Reddit fan theory explains, Batman could knock the mask off one of these henchmen in the middle of battle, causing him to breathe Scarecrow's fear-inducing toxin gas and altering his psyche.
Deeply affected by the fear gas and possibly suffering from amnesia, this member of the League of Shadows could escape the authorities, leave the League, and take on a new persona to wage war against the city he couldn't destroy and the man who instilled fear in him. As the Joker, he could defect the League of Shadows and prepare his own plan to break Gotham apart, learning from Ra's Al Ghul and Scarecrow's mistakes and using his League of Shadows training to put his plan in motion. Feats like infiltrating the hospital where Harvey Dent is being treated and filling it with bombs are easy tasks for a former League of Shadows warrior.