The Akumaizer are the main antagonists of Movie War Ultimatum. They are the last remnants of the Akuma Clan from the Demon World that were seen by humans of several thousand years ago as "devils" and were pushed underground. They receive five Gates from the Phantoms and make the Infinite Monster Plant which revives the Dopants, Yummy, and Zodiarts that were previously defeated by the Kamen Riders to make a Monster Army to invade and conquer humanity.
Each of the Akumaizer wield a rapier-like weapon called the Jankel. They can also perform a group finisher called the Devil Team Attack.
- Xatan is the leader-like figure of the Akumaizer who is able to use the Underworld's magic. He plans on taking over the world together with his subordinates Eel and Gahra. He is destroyed by Wizard's Special Dragon Rush.
- Eel is the most agile and skilled fighter of the Akumaizer, skilled in aerial combat. He is destroyed by Fourze's Rider Super Galaxy Finish.
- Gahra is the strongest of the Akumaizer. He has a large, muscular physique and has superhuman strength. He is later destroyed by Wizard's Double Flame Slash Strike from sacrificing himself to save Xatan.
- Akumaizer are same as Space Ironmen Kyodain, when they were created by Shotaro Ishinomori as heroes of their respective shows. Whereas their modern counterparts are evil re-imaginings of those characters made to antagonize the Kamen Riders.
- The Akumaizer are a homage to the protagonists of Akumaizer 3, which was also created by Shotaro Ishinomori.