As Harley Quinn took complete control of the security systems and Batman was forced to stay behind by the Arkham guards out of fear that his presence would unsettle the more violent prisoners, Joker violently murdered his escorts and escaped into the heart of the asylum, slaughtering dozens of guards and inmates alike and released large quantities of his laughing toxin into the air ducts.
Believing that Poison Ivy could further strengthen his Fear Toxin, Scarecrow tasked Harley Quinn and Penguin with freeing her from the Bludhaven Police Department. To become a psychiatrist, you would need to complete medical school in order to earn an M.D. Scarecrow dragged into the river by Croc. The fact that Scarecrow chose Halloween to attack Gotham further showed his fascination of fear, and why he chose to take advantage of the night that lore had said fear would be at it most powerful, since monsters, like Scarecrows, are said to be at their strongest on Hallow's Eve, which may explain how Scarecrow became more dangerous. Scarecrow revealed in the chaos as all of the remaining criminals, except for the Arkham Knight's Militia, began attacking and killing each other in blind fear. After Crane was ultimately exposed and captured by Batman and sent to the same institution that he used as a staging ground for his inhumane experiments, he realized that the hero had proved to be the only individual who was capable of inflicting fear on him. Scarecrow injected Batman with even more Fear Toxin, which leads up to another giant Scarecrow hallucination, in which Scarecrow watched as Batman fought his army of skeletons, including two larger ones (actually Joker's Henchmen, and two Titan Henchmen). Instead of a hat, he chose a hood combined with a gas mask.
As Batman examined a Titan-mutated plant, Scarecrow warned him to stay away, or he would drop the bag of Fear Toxin into the underground river; the cave would fill with all Batman's worst nightmares. Scarecrow took Gordon hostage once again and left in a militia aircraft, while he ordered a squadron of tanks to pin Batman down in the meantime. Crane rode the elevator all the way down to the lowest level, to reach the sewers; he didn't know that the sewer system served as the lair of Killer Croc. Scarecrow was one of the Arkham inmates on the Joker's "party list", and who was released during the Clown Prince of Crime's takeover of Arkham Island, both to distract Batman and for the Joker's general amusement as Crane's experiments often left his victims dead of heart attacks. Even worse, Batman learned that Scarecrow's bomb that was to cover the entire east coast in fear toxin was merely the first phase of his operations, and, in the end, the deranged villain intended for the entire world to feel the effects of his toxin.
Young believed that the chemicals derived from Venom could serve as the perfect foundation for her own serum. Having been exposed to his toxins, the inhabitants of Gotham have now seen Batman wasn't unstoppable and had finally failed them. While he failed to emotionally break him and plunge Gotham into terror, he succeeded in exposing the Dark Knight's identity to the world which forced Bruce Wayne to seemingly kill himself to protect his loved ones.
After he arrived at the abandoned and destroyed Arkham Asylum, that was converted into a base of operations by Scarecrow, Batman was restrained and put in front of a camera, as Gordon and Robin watched from the side, that broadcasted to the entire world. While he was a giggling psychopath in Asylum, Scarecrow was an emotionless mastermind who only laughed twice. Scarecrow escaping in Batman: Assault on Arkham.
Crimes Several fear gas tanks can be found around Arkham City.