Fear Toxin, also known as Fear Gas, was the signature weapon used by the Scarecrow. It induced involuntary and sometimes irrational fear in its victims' minds, driving them insane.
While Scarecrow never appeared, a version of his Fear Gas was used by the outlaw Shame. Here instead of showing a person's worst fears, it caused the person to become a coward towards everything. It eventually wears off.
The Scarecrow used his Fear Toxin in the Batman: Animated Series in all of his major schemes. In the animated series the gas was coloured red (possibly to make it distinguishable from the Joker's green laughing gas). In his first appearance, the Scarecrow simply uses it as a weapon to protect himself or ward off interferences (by spraying it through vents in the fingers of his gloves). Batman, however, finds his lair and opens the tanks containing the toxin, exposing Scarecrow to his own gas and showing him his own fear of bats. Batman sends him to Arkham Asylum. When Scarecrow returns, he makes the gas into a powder that he uses to rig some sports games and collect bid money to fund his "experiments." He attempts to drop a vial onto the audience in the stadium, but Robin catches the vial and Scarecrow is captured by Batman. In his next appearance, Scarecrow exposes Batman to a large dose of the gas (causing him to almost lose his mind). While Batman struggles to get a grip on reality, Scarecrow plans to vent the fear toxin into the city's water supply. Batman shows up and stops Scarecrow's men, but Scarecrow hits the lever and runs. Batman, however, manages to stop the machine before it can vent the poisonous gas and the whole thing explodes. Scarecrow attempts to escape but is exposed to all the gas that escapes from the destroyed machine. He is then sent back to Arkham, scared out of his wits.
- "How are you doing this? You've ingested enough toxins to drive 10 men insane. What are you?"
- ―Scarecrow shocked at Batman's amazing resilience to his toxin[src]
Also through one of the Riddler's riddles, you find a few canisters of Fear Gas which make its victims go into utter fright, terror, horror, and panic.
The Fear Toxin is the main part of Ra's al Ghul's master plan to destroy Gotham. The gas is actually made from the Blue Poppies near Ra's al Ghul's dojo. Dr Jonathan Crane is the corrupt chief therapist at Arkham Asylum, he has a vast amount of knowledge in science due to working with medicine for his patients. Crane makes an alliance with Ra's al Ghul and Falcone and uses the flowers to make his deadly fear toxin. Crane also has Falcone sneak in necessary ingredients for the toxin. Crane tested the gas on his patients while doing so he creates a special mask that resembles that of a scarecrow. Crane keeps a large amount of the gas for himself and uses it through special gas sprayers he keeps on his wrists.
Crane puts the final steps of the plan into action when Ra's al Ghul's men deliver him a special Wayne Industries device that can convert liquid into gas. Crane has a liquid form of the fear toxin poured into the water supply and it is soon activated on a moving train, gassing the parts of Gotham it passed and putting all citizens caught in it into a state of panic. Crane escapes from Arkham after a defeat from Batman (where he was stripped of his mask and sprayed with his own toxin which destroyed the rest of his sanity) and rides through the streets on a horse, terrorizing the citizens of Gotham. Scarecrow, however, is forced to retreat after being shot in the face with a taser gun by Rachel Dawes. Batman manages to make a cure for the toxin and it is reproduced to cure the victims poisoned by Crane's gas. The toxin is later halted and flushed out of the water supply.
Scarecrow uses his fear toxin to keep people in constant fear of him. He injected Killer Croc with the toxin so that it coursed throughout his body and Croc could pass the toxin on by biting people.
The Fear Gas is only used briefly in this movie. Scarecrow had apparently kept a large amount of toxin for himself and even tried to sell it to street gangs. During a meeting with one of his buyers over the unsatisfactory results of his toxin, they are interrupted by some false Batmen, one attacks Scarecrow and he repels him by spraying him with the toxin. After the real Batman shows up, he captures the gangsters and eventually the escaping Scarecrow.