- "Now, I know what you're all thinking. Why have I gathered this legion of Horribles? That has a nice ring to it."
- —Jerome Valeska
The Legion of Horribles was a terrorist organization formed by group of escaped Arkham Asylum inmates;Jerome Valeska, Oswald Cobblepot, Jervis Tetch and Jonathan Crane, later reinforced with Victor Fries, Bridgit Pike and Butch Gilzean, which operated in Gotham City with the goal to drive the city into madness. The Legion was named and lead by Jerome Valeska.
- Jerome Valeska (leader; deceased)
- Oswald Cobblepot/The Penguin (defected)
- Jervis Tetch/Mad Hatter (formerly)
- Jonathan Crane/Scarecrow (formerly)
- Bridgit Pike/Firefly (formerly)
- Victor Fries/Mr. Freeze (formerly)
- Butch Gilzean/Solomon Grundy (deceased)
- "A Dark Knight: Mandatory Brunch Meeting"
- "A Dark Knight: That's Entertainment"
- "A Legion of Horribles" was the title of the penultimate episode of season 2.
- Season 4 loosely adapts Batman: Year One and The Long Halloween. Cameron Monaghan hinted on Twitter that the Legion of Horribles serves as an homage to the villainous roster from The Long Halloween, composed of the Joker, Penguin, Catwoman, Scarecrow, Solomon Grundy, Mad Hatter, and Poison Ivy. Most of these villains notably appeared as members of the Legion.