Fear Toxin (Earth-6531)

"Welcome to my world."

- - Scarecrow to Batman

The Fear Toxin is a hallucinogen chemical created by Doctor Jonathan Crane alongside the League of Shadows, which is capable of inducing individuals to witness by their biggest fears and even lead to death.

History
Doctor Jonathan Crane was approached by the mysterious benefactor Ra's al Ghul, who, inspired by Crane's experiments, offered him to test his proposed drug on the entirety of Gotham City and be rewarded beyond imagination. Because he was led to believe Ra's only wanted to hold Gotham for ransom, Crane agreed to help him, and was able to weaponize the compound found at the League of Shadows' blue flowers, thus creating the Fear Toxin, a hallucinogen drug capable of inducing nightmares into others.

Manipulating the crime lord Carmine Falcone into smuggling the Toxin's ingredients through Gotham, Crane helped him by pledging the insanity of Falcone's henchmen so they could go to Arkham Asylum, Crane's workplace, and help him developing the drug at a secret underground laboratory.

Their plan eventually was reached when al Ghul and his assassins spread the hallucinogen through The Narrows, the slums of Gotham, reducing all of its inhabitants to mindless savages afraid of others, whom they perceived as demons and monsters. They also released several lunatics and criminals from Arkham and Blackgate Penitentiary to the location to spread more chaos. Inside the Gotham Monorail, Ra's planned to have the train, full of Fear Toxin, crash at Wayne Tower, and release the toxin all over the city to finally destroy it.

The Toxin was eventually halted when Batman, with the help of Sgt. James Gordon, was able to stop the League's plans, which ended killing al Ghul and his followers, but saving Gotham in the process.