Downfall is a terrorist organization bent on world domination and the main antagonistic faction of the 2019 film Kamen Rider Build NEW WORLD: Kamen Rider Grease.
They are led by Simon Marcus, a Deputy Officer of the United Nations, and their personnel includes mad scientist Keiji Uraga. They emerged some time after Killbus was defeated and the rest of Team Build got their memories restored.
During a meeting between Simon Marcus and officials of the Japanese government to discuss the Rider System, Downfall members led by Keiji Uraga staged an attack. Revealing himself as Downfall's leader, Simon Marcus defeated Kamen Rider Rogue and took the Japanese government building hostage.
Downfall then sent Phantom Crushers to track down and disable the transformation abilities of the other Kamen Riders. However, with assistance from the New World Takumi Katsuragi, Sento Kiryu was able to develop a concoction from Nebula Gas that restored the Riders' ability to transform.
The Kamen Riders then attacked Downfall and defeated all of their members, liberating Japan from their grasp.