The Another World Riders are Dark Kamen Riders from alternate worlds and antagonists in the final arc of Kamen Rider Zi-O. They are summoned from Another Worlds created by Another Decade where, due to a slight change in events, they prevailed over their respective heroic Kamen Riders and succeeded in their goals.
After obtaining the powers of Decade, Swartz transformed into Another Decade and began abducting various people to transport to Another Worlds so he could replace them with his Another Riders. Swartz thus summoned Kamen Riders G4, Rey, Dark Ghost and Fuma to fight Zi-O and Geiz. The four Dark Riders overwhelmed them, but before they could be defeated they were swooped away by Woz with his scarf.
Confronting Sougo again, Swartz brought forth Katsumi Daido/Kamen Rider Eternal as well to fight Sougo Tokiwa, though he ended up being preoccupied by Michal Minato/Kamen Rider Aqua. Swartz later summoned White Woz and Kamen Rider Yuuki as well to deal with the Riders, but White Woz ended up defecting to Zi-O's side in order to save Geiz, who had been transported to the Another World by Swartz. White Woz thus conspired with his "black" counterpart and Sougo to devise a plan to destroy the Another Worlds and free Geiz.
After getting Eternal to use his finisher, Sougo transported both himself and Eternal to the Another World where Eternal's Rider Kick destroyed some of the Another Worlds and freed their prisoners, including Geiz. Because their Another Worlds were destroyed, Katsumi, White Woz and Yuuki all disappeared, though Swartz was unfazed as he still had G4, Rey, Dark Ghost and Fuma.