This is a Blu-Ray exclusive miniseries for Kamen Rider Saber.
Part 1 takes place at different points in time: The opening scene takes place 17 years prior to the series. The first half of the episode takes place approximately 15 years prior to the series at the height of the war of Wonder World, with more focus on the perspectives of Daichi Kamijo and Hayato Fukamiya. The second half comprises scenes up to the events of Episode 16 from the perspectives of Daichi, Master Logos, and Reika Shindai.
Source: Kamen Rider Fandom)