Similar to Kamen Rider Decade Complete Form, he can summon his predecessors in battle, but unlike Decade Complete Form, he can summon them in their different forms and he can summon multiple past Riders at the same time. As seen in these images, Grand Zi-O summons OOO Gatakiriba Combo, Kuuga Pegasus Form, Build Genius Form, and Gaim Suika Arms in battle. In another image, he summons Build RabbitTank Form and Kuuga Mighty Form in battle in his fight against Another Den-O.
Finally, here are the episode summaries of Kamen Rider Zi-O's Den-O Arc:
- Episode 39 (airing on June 9th) - After undergoing an emergency landing, Geiz and Tsukuyomi meet Momotaros, Urataros, Kintaros, and Ryutaros in a crashed DenLiner. The Imagin asks the two to help them repair the train of time.
- Episode 40 (airing on June 23rd, the series will take a break on June 16th) - After it was revealed that Another Den-O hijacked the DenLiner back in 2017, Sougo proposed that they head back to the past to prevent it from happening.
[Source: HeroShock, SpecialForm12]