The Crane Serum is a powerful hallucinogenic compound which was created by the late Gerald Crane, and is utilized by various criminals in Gotham City (most notably by Gerald's son, Jonathan).
After being paralyzed with terror during the burning of his house (which led to the death of his wife), Dr. Gerald Crane became obsessed with overcoming fear, and thus, began harvesting the adrenal glands (most notably, cortizal and adrenaline) of people in high-intensity fear situations (killing them with their own phobias, in order to maximize their state of terror at the moment of death) to create this serum which forces the subject to see their greatest fears, in the hope that seeing said fears would allow them to face and overcome said fears.
Upon completion, Crane took the serum himself, and became completely fearless as a result of overcoming his fear, ultimately resulting in his death when he initiated a shootout between himself, Harvey Bullock and James Gordon. As shown with Crane's son, Jonathan, a high enough dosage of the serum could force the subject to see their fear constantly.
The serum is later recreated when Grady Harris and Merton's team forced Jonathan Crane into making a new batch for them to use for crimes. Crane updated the serum's formula and made it into a gaseous version. Jonathan eventually uses the formula himself, upon donning the moniker 'The Scarecrow'. Upon entering Gotham Asylum, Scarecrow gassed Warden Reed and the inmates of Arkham Asylum. Jonathan then sprays Gordan with the gas and attempts to get him to commit suicide, but fails.
Upon exposure, either through injection or inhalation, the Crane Serum causes its victims to experience hallucinations of either their deepest fears, or their own inner demons (as shown with Gerald). Eventually the effects will wear off and the victim will revert back to normal (albeit with some differences, as Gerald became completely fearless following his inoculation)
Anyone who is exposed to larger-than-average doses of the serum will still see their greatest fear/inner demons through hallucinations, but twisted into a much darker, more demonic form. Prolonged exposure can induce minute brain damage, placing the victim in a constant state of intense terror. Victims such as Jonathan Crane and Warden Reed, who were exposed for an extended period of time, become what they fear. As shown with Gordon though, people with strong enough willpower can resist the effects, a feat which Jonathan claimed to be impossible to the point that "[one] can't simply stop being afraid, [one] can only become [their] demon".
People exposed to the Fear Toxin
- Gerald Crane (Injected into himself; removed his sense of fear)
- Jonathan Crane (Injected by his father; overloaded then increased his sense of fear)
- Oswald Cobblepot ("rehabilitated" by Hugo Strange)
- Unnamed Banker (Sprayed by Merton; caused him to hallucinate fiery demons)
- Oswald Cobblepot (Sprayed by Merton; caused him to hallucinate a demonic Riddler, unfrozen)
- Grady Harris (Sprayed by Jonathan Crane)
- Warden Reed (Sprayed by Jonathan Crane; caused him to see demonic clowns)
- Unidentified Gotham Asylum Inmates (Sprayed by Jonathan Crane)
- Jim Gordon (Sprayed by Jonathan Crane; caused him to see Leslie Thompkins dying in sorrow)
- Unidentified Arkham transport driver (Sprayed by Jonathan Crane)
- Unidentified Blimp pilot (Sprayed by Jonathan Crane)
- Unidentified Jeremiah Cultist (Sprayed by Jonathan Crane; inadvertently)
- Bruce Wayne (Sprayed by Jeremiah Valeska and Jonathan Crane; caused him to see Alfred Pennyworth tortured)
- Unidentified Drug maker (Sprayed by Jonathan Crane)
- Unidentified GCPD cop (Sprayed by Jonathan Crane)
- The Crane serum is based off the fear gas that Scarecrow uses in the comics.
- Interestingly, this version of the Scarecrow's Fear Toxin bears similarities to the "Super Toxin 451-A" from Batman: the Dark Knight (Volume 2).
- Curiously, Reed was exposed to this serum for a far shorter period of time (in comparison to Jonathan, who suffered under its influence for three years) before becoming one with his fear.
- In a scene ultimately deleted from the final version of "Legend of the Dark Knight: Year Zero", Lucius Fox was originally going to be sprayed with fear gas by Scarecrow. This scene was included as a special feature on the later DVD and Blu-ray release. For a more detailed description of this scene, see Deleted scenes#Season 5.
- Had the scene been included, Lucius would have been the final character on the show to be exposed to the fear toxin.
- Stephens, John (writer) & Behring, John (director) (February 2, 2015). "The Fearsome Dr. Crane". Gotham. Season 1. Episode 14. FOX.
- Woodruff, Ken (writer) & Copus, Nick (director) (February 9, 2015). "The Scarecrow". Gotham. Season 1. Episode 15. FOX.
- Behring, John (writer) & Harper, Jordan (director) (March 14, 2016). "Wrath of the Villains: This Ball of Mud and Meanness". Gotham. Season 2. Episode 14. FOX.
- Stephens, John (writer) & Cannon, Danny (director) (September 21, 2017). "A Dark Knight: Pax Penguina". Gotham. Season 4. Episode 1. FOX.
- Cannon, Danny (writer) & Milito, Lous Shaw (director) (September 28, 2017). "A Dark Knight: The Fear Reaper". Gotham. Season 4. Episode 2. FOX.