A sub-series in the long-running Ultra Series, which began in 2007. Has an eye towards fan service of some capacity, and elements from it often cross over into and linking together main installments in the franchise. The main domain of Ultraman Zero, who serves as the sub-series' perennial protagonist, who would also host Ultraman Retsuden and Shin Ultraman Retsuden from 2011 to 2016 (and these would be followed up by Ultraman Zero: The Chronicle from 2017, which recounts the main entries he had appeared in, all while still hosting, Ultraman Chronicle Zero & Geed from 2020, which follows it up while being hosted by Asakura Riku, Ultraman Chronicle Z: Heroes' Odyssey from 2021, and Ultraman New Generation Stars from 2023 onward).
The sub-series can be subdivided into two main eras; the first era which sees the rise of the Reionics, with Reiblood Seijin and Ultraman Belial acting as its main antagonists, and the second era which sees a greater focus on multiversal stories, culminating in the emergence of the Absolutians as its main antagonists.
