The Divine Key made from the core of the Previous Era's Herrscher of Reason and thus capable of creating replicas of weapons. Originally a gold cube, it fell into the hands of Otto Apocalypse, and took control of one of his Soulium bodies after his death. In the latter form, it was responsible for bringing the Sky People to Earth and causing the St. Fountain Incident depicted in A Post-Honkai Odyssey.
Its subsequent status is unknown, but given its appearance in Welt Yang's Honkai: Star Rail official art (post #5615313) and Welt having flashbacks of it upon seeing Luocha, its future return is likely.
