Fear Toxin is a chemical compound that was created by Dr. Jonathan Crane, which he used as Scarecrow.
It was a drug that could be either inhaled or injected directly into the blood stream. When ingested, it induced intense and sometimes irrational fear, which caused the user to go insane, or even die in larger doses. Death is most likely caused by an extremely concentrated dosage. The cause of death is most likely to be of swift and sudden heart attacks or some unknown effect the chemical has on the brain. Heart attacks are probably the most common cause of death as Fear Toxin would cause heart rates to increase exponentially and lead to cardiac arrest.
Arkham Asylum Incident
In case of the event of his capture, Scarecrow kept a cache of Fear Gas hidden behind a wall at Arkham Asylum. When Scarecrow escaped from his cell, he tapped it to fight Batman. He used it 3 times against the hero. First, he released it into an elevator at the Medical Facility. Second, he released it through a vent in the wall in the Library at the Arkham Mansion. The final time, he released it through a crack in a pipe in the Cell Block Transfer at the Intensive Treatment Center. Scarecrow planned on releasing it in the Gotham River, in Killer Croc's Lair, but he was pulled underwater and mauled by Killer Croc.
Arkham City Incident
Though Scarecrow did not appear, his Fear Gas was seen throughout Arkham City. Fear Gas Canisters appeared at the third Riddler Hostage Room and also at Hush's Hideout. A victim of the gas could be found in Scarecrow's Boat. He had a spasm when Batman approached him.
Arkham Knight Incident
Scarecrow planted bombs full of Fear Gas all over Gotham City, and forced a citywide evacuation. Most, if not all, of the Thugs and criminals there wore gas masks in an attempt to protect themselves if Scarecrow decided to detonate the bombs. Later in the game, the Arkham Knight's Militia went over the plan to evacuate Gotham when the bomb went off, and were reminded that it was absorbed through the skin. The only way to avoid the gas was to be underground or high above it. Simon Stagg was later revealed to have helped Scarecrow make that new strain of Fear Toxin, and who saw it as profitable. Exposure to the Fear Toxin also caused Batman to have hallucinations about the Joker, who taunted him about succumbing to the blood infection and becoming him. According to Stagg's research log, the gas was potent enough that not even gas masks could block out the effects.
Arkham Asylum Incident
After being exposed to the Fear Gas while he walked the halls of Arkham Asylum, Batman experienced elaborate hallucinations- ranging from finding his parents' bodies in the Arkham Asylum Morgue, reliving his last moments with them as they lived, and an 'inversion' where he was the lunatic who had been brought into Arkham Asylum by the Joker, which culminated in him being forced to travel through a twisted, nightmare-esque version of Arkham Asylum while evading a giant Scarecrow who would crush him if he was caught in his gaze.
These confrontations ended when Batman shined the Bat-Signal on Scarecrow at the end of the 'maze'; the Signal apparently represented Batman's strength of will as he focused to bring himself out of the hallucination. Scarecrow himself noted - after the third and final confrontation - that Batman continued to resist even after he took in enough toxin to drive ten men insane.
Arkham Knight Incident
After being exposed to Scarecrow's Fear Toxin again, Batman experienced horrifying flashbacks of the Joker's past heinous crimes (such as the crippling of Barbara Gordon, and the torture and "death" of Jason Todd), as well as a hallucination of the Joker himself, who taunted the Dark Knight throughout the main story. Along with this, Batman experienced vivid hallucinations where Barbara Gordon committed suicide (being realistic enough to even fool Detective Mode), snapping Joker's neck, and where he became a mass-murderer like Joker. Unlike the hallucinations in Arkham Asylum, there was no definite way to break free from them, as most of them were cinematic in nature and did not offer much in the way of gameplay. However, if one had access to the bonus Scarecrow Nightmare Missions (a DLC for the PS4 version of Batman: Arkham Knight), Batman, under the influence of Fear Toxin, could be thrown into battle with a giant version of Scarecrow and Militia Drones.
Batman Arkham Asylum
Batman: Arkham Knight
- The Joker
- Hugo Strange
- Giant Scarecrow
- Thomas & Martha Wayne
- According to Scarecrow, he has the needles to inject toxin because in it's purest form, the fear toxin will leaving much more damage from the trauma. The gaseous version doesn't have that much effect, as it can be cured and the trauma treated.
- One of the Riddler's riddles involved the player finding a few canisters of Fear Toxin at Arkham West on Arkham Island.
- When under the influence of Fear Toxin in Arkham Asylum, Batman's eyes glowed orange. In Arkham Knight, they instead glowed green, which alluded to Joker's DNA in his blood.
- How Batman's eyes glowed differed between the games (due to Knight being on a more advanced system than Asylum); in the first one, Batman's eyes glowed similar to Detective Mode, while only the irises of his eyes changed color in Knight.
- In Knight, Batman's veins darkened when infected with the Fear Toxin. This visual effect is also present when Joker temporarily takes over Batman's body on the airship.
- The third encounter with Scarecrow, in which the game appeared to reset then showed a false Game Over screen was most likely a reference to the Metal Gear Solid games, or Eternal Darkness, when Psycho Mantis/losing your sanity made the game "Crash". It could also be a reference to Earthbound as the screen blacked out after the game "Crashed", much like when Giygas was defeated. It could also be a play on a gamer's worst fear; their game crashing before they could save.
- The skeletons that Batman saw in the Nightmares appeared to be normal enemies in reality. This was why when Batman woke from Scarecrow's Final Nightmare, he found many knocked out Joker's Henchmen and a pair of Titan Henchmen that resembled the skeletons that he fought in the Nightmare.
- In the Nightmare section where you hid from Scarecrow's gaze, when the lightning flashed, Batman appeared to be Scarecrow.
- If you tried to use Detective Mode while you were under the effect of Fear Gas, Scarecrow did a very quiet laugh, as if the button triggered a taunt.
- In Arkham City, some of Scarecrow's canisters could be found at the third Riddler Hostage Room as well as at Hush's Hideout. This foreshadowed Scarecrow working with Hush and Riddler behind the scenes.
- This was also likely in reference to the trio's partnership in Batman: Hush. According to Riddler in the comic, Scarecrow was in fact the one who gave Hush (who he had also mentored when he was young) his name by singing Hush Little Baby.
- The first Nightmare seemed to be Batman's worst fear: he had no assistants, and that he had failed to save the people that were close to him.
- The second Nightmare was Bruce Wayne's worst nightmare: having to relive his parents' deaths in Crime Alley again.
- The third Nightmare was the player's worst nightmare, like mentioned above: The game freezing without saving and then restarting.
- Scarecrow's new strain of Fear Toxin in Arkham Knight was apparently strong enough to effect those who were usually resistant or immune to toxins. Poison Ivy revealed that she had immunized herself against it.
- In Arkham Knight, Simon Stagg reveals, in his research notes, aboard one of his airships that Fear Toxic could be used as a possible cure for depression. The Fear Toxin would most likely alter the person's brain chemistry but it is unknown if Fear Toxin itself would cure depression if applied directly.
- Therefore, if Fear Toxin does not cure depression directly, then the chemical's formula could most likely be altered into a different substance with a different or even positive effect on the human mind.
- If the ending in Arkham Asylum where Scarecrow grabs the floating Titan crate is accountable in canonical status, perhaps he adapted the formula to make improvements to his Fear toxin (which would partially explain why it was more powerful in Arkham Knight).