Fear Toxin

Fear Toxin, also known as Fear Gas is a chemical compound created and used by Scarecrow.

About
It was a drug that could be either inhaled or injected. When ingested it induced intense and sometimes irrational fear, which caused the user to go insane or even die in larger doses. Death is possibly by a concentrated dosage and the cause of death is most likely to be of sudden heart attacks, self-harm through use of the environment or unknown effects the chemical has on the brain and other internal organs. Heart attacks are probably the most common cause of death as a dose would cause heart rates to increase exponentially and lead to cardiac arrest. When Batman was exposed to the Fear Toxin in-game it led to a Scarecrow Nightmare.

Arkham Asylum Incident
In case of the event of his capture Scarecrow kept a cache of Fear Gas hidden behind a wall in Arkham Asylum. When Scarecrow escaped from his cell he tapped it to fight Batman. First he released it into an elevator at the Medical Facility, second through a wall vent in the Arkham Mansion's library and the final time through a cracked pipe in the Intensive Treatment Center's Cell Block Transfer. Scarecrow planned on releasing more of it into the Gotham River in Killer Croc's Lair but was pulled underwater, mauled and disfigured by Killer Croc.

Arkham City Incident
Scarecrow's gas was seen throughout Arkham City. Canisters of it appeared in the third Riddler Hostage Room and also at Hush's Hideout. Roach, a victim of the toxin could be found in Scarecrow's Boat surrounded by crates of cockroaches used for Scarecrow's experiments and had a spasm when Batman approached him.

Arkham Knight Incident
According to a City Story unlocked by scanning a Riddle, Scarecrow ground up the cockroaches he kept in his boat and forced Roach to consume them as part of his tests; the results were apparently promising enough that Scarecrow used the insects as one of the ingredients in his new toxin.

Scarecrow supposedly planted bombs of Fear Toxin all over Gotham City and forced a citywide evacuation after Jason Todd released some of it at Pauli's Diner. After seizing control of Ace Chemicals, he had his toxin produced on a larger scale and the factory rigged to blow. Most, if not all remaining occupants of Gotham wore gas masks and similar equipment to protect themselves if Scarecrow detonated his bombs. The Arkham Knight's Militia went over the plan to evacuate or find cover when the bombs went off and were reminded that it was absorbed through the skin, meaning the only way to avoid the gas was to be underground or above it. Simon Stagg was later revealed to have helped Scarecrow make the new strain of Fear Toxin while also using the Cloudburst invented by Alex Sartorius and saw it as profitable. Exposure to the Fear Toxin also caused Batman to have hallucinations about the the Joker, who taunted him about succumbing to his previous blood infection and becoming him. According to the Stagg Enterprises research logs the gas was potent enough that not even facial and bodily protection could block out the effects and, according to a militia soldier only slowed it down for a few seconds before kicking in.

Scarecrow's new strain was apparently strong enough to affect those who were usually resistant or immune; Poison Ivy immunized herself using spores she produced but removing the Cloudburst-amplified toxin resulted in her death, and despite Batman's willpower he was weakened to the point of almost letting Joker take control of him.

Arkham Asylum Incident
After being exposed to the gaseous toxin while walking through the halls of Arkham Asylum, Batman experienced elaborate hallucinations ranging from finding his parents' bodies in the Morgue, reliving his last moments with them and an 'inversion' where he was the lunatic brought to Arkham by the Joker which culminated in him being forced to travel through a twisted, nightmare-esque version of Arkham composed of islands floating over a brown whirlwind while fighting skeletons and evading a giant Scarecrow who normally crushed him if caught in his gaze.

The confrontations ended when Batman shone the Bat-Signal on Scarecrow at the end of each 'maze'; the signal apparently represented Batman's strength of will as he focused to get out of the hallucinations. Scarecrow himself noted after the third and final confrontation that he continued to resist even after taking in enough toxin to apparently drive ten men insane.

In the DLC content of Arkham Asylum and Batman: Return To Arkham, Batman could enter a Scarecrow Nightmare where he needed to avoid being hit for as long as possible.

Arkham Knight Incident
After being exposed to Scarecrow's toxin again, Batman experienced horrifying flashbacks of Joker's heinous crimes (the crippling of Oracle and torture of Jason) as well as an auditory and visual recreation of Joker who taunted the Dark Knight. Along with this, Batman experienced vivid hallucinations where Oracle committed suicide (apparently being realistic enough to even fool Detective Mode), he snapped Joker's neck while fighting him in Crime Alley and essentially became Joker himself. Unlike the hallucinations in Arkham Asylum there was no definite way to break free as most were cinematic in nature and did not offer much in the way of gameplay. Unfortunately, exposure to the toxin caused Joker to see his fear of being forgotten letting Batman retake his being, and Scarecrow was not immune to his latest batch of toxin causing him to go mad.

However, if one had access to the Scarecrow Nightmare Missions DLC for the PS4 version of Arkham Knight, Batman with the Batmobile could be thrown into battle with a giant version of Scarecrow and armies of the Arkham Knight's Drones.

Batman: Arkham Asylum

 * James Gordon
 * Thomas and Martha Wayne
 * Joe Chill
 * Scarecrow
 * Crime Alley
 * Arkham Asylum
 * Skeletons
 * The Joker
 * Victor Zsasz
 * Harley Quinn
 * Scarface
 * Aaron Cash
 * Quincy Sharp
 * Penelope Young
 * Other Arkham staff
 * Arkham Lunatics

Batman: Arkham Knight

 * Demons
 * The Joker
 * Oracle
 * Jason Todd
 * Monarch Theatre
 * Crime Alley
 * Thomas and Martha Wayne
 * Penguin
 * The Riddler
 * Two-Face
 * Killer Croc
 * Hanrahan
 * James Gordon
 * GCPD Building
 * Bleake Island
 * Alfred Pennyworth
 * Harley Quinn
 * Batman
 * "Ghost"
 * Scarecrow
 * Arkham Knight's Drones
 * Miagani Island
 * Founders' Island
 * Skeletons

Trivia

 * According to Scarecrow himself, he has the needles used to inject toxin because in its purest form it leaves more damage from the trauma. The gaseous version doesn't have much effect as it can be cured and the trauma treated.
 * One of the Riddler's Riddles involved finding canisters of Fear Toxin in Arkham West.
 * When under the influence of the toxin in Arkham Asylum, Batman's eyes glowed orange which was a nod to the color of the chemical. In Arkham Knight they instead glowed green which alluded to Joker's infected DNA in his blood.
 * How Batman's eyes glowed differed between games due to Arkham Knight being on a more advanced system than Arkham Asylum; in the former Batman's eyes glowed similar to Detective Mode while only the irises of his eyes changed color in the latter.
 * In Arkham Knight, Batman's veins darkened and his face became similar to after he was poisoned by Joker in Arkham City. This effect is also present when Joker takes over Batman's body on one of Stagg's Airships and before Batman fights back the infection.
 * In Arkham Asylum, when light briefly flashes Batman is replaced by Scarecrow himself before a second flash returns him to normal.
 * The third encounter with Scarecrow in which the game appears to reset and show a false Game Over screen was most likely a reference to games like ' or ' when triggering an event made the game "crash". It could also be a reference to  when Giygas is defeated. It could also be a play on a hardcore gamer's worst fear: their game crashing before they could save.
 * The skeletons in the Nightmares appeared to be normal enemies in reality. This was why when Batman awoke from Scarecrow's last Nightmare in the Asylum he found Joker's Henchmen and a Titan Henchman resembling the skeletons he fought knocked out.
 * The demons from the Nightmare at Pauli's Diner are actually people also under the toxin's effects; this is proven through character dialogue if the player as Officer Owens decides to shoot any of them.
 * If you tried using Detective Mode while fully under the toxin's effects in Asylum, Scarecrow laughed quietly as if the button triggered a taunt.
 * In Arkham City, some of Scarecrow's canisters could be found in the third Riddler Hostage Room as well as 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 the one who gave Hush (who he mentored) his name by singing the nursery rhyme Hush Little Baby.
 * The first Nightmare seemed to be Batman's worst fear: he had no one beside him and failed to save those closest to him.
 * The second Nightmare was Bruce Wayne's worst fear: having to relive the deaths of his parents.
 * The third Nightmare was the player's worst fear mentioned above: the game freezing and restarting without saving.
 * In Arkham Knight, Stagg reveals in his research notes that the toxin could be used as a possible cure for depression or to turn an army against itself. The Fear Toxin would most likely alter a person's brain chemistry but it is unknown if the toxin itself would cure depression if applied directly.
 * Therefore, if it doesn't cure depression directly then the formula could be altered into a different substance with different effects on the human mind.
 * If the ending in Arkham Asylum where Scarecrow grabs the floating Titan crate is canonical, perhaps he adapted the formula to make improvements to his toxin (which would partially explain why it was more powerful in Arkham Knight).
 * In Arkham Knight, while exploring Ace Chemicals before coming across another deceased worker you can find a chemistry table with a vial labeled "Copperhead Poison", implying Scarecrow used the assassin's signature toxins to enhance his own; Copperhead's poison was also shown to cause hallucinations that could either be cured and/or broken out of.