Scarecrow uses fear toxin in Christopher Nolan’s The Dark Knight.
Dr. Jonathan Crane, played by Cillian Murphy, is the villain known as Scarecrow. In the film, he works with the mob and uses a hallucinogenic fear toxin that causes victims to experience terrifying visions. The weapon is part of the broader criminal plot involving the Joker and Gotham City.
The toxin connects The Dark Knight to Batman Begins, where Crane first appears as a major antagonist. In Batman Begins, he uses the compound in a plan to contaminate Gotham’s water supply and disperse it through the city’s air. The Dark Knight presents Crane in a smaller role, but the character’s signature chemical remains recognizable.
The answer is often shortened to “fear gas,” especially in comic-book discussions. Nolan’s films commonly describe it as fear toxin, making that the most precise canonical answer for the question.