Century King Shadow Moon (世紀王シャドームーン, Seiki Ō Shadō Mūn) is a character who serves as the primary antagonist in Kamen Rider Black and Kamen Rider Black RX. He is Kohtaro Minami's archenemy.
Shadow Moon is the alter ego of Nobuhiko Akizuki (秋月 信彦, Akizuki Nobuhiko) who had been abducted by Gorgom to become one of their new kings. Shadow Moon is both the stepbrother and best friend to Kohtaro Minami, who had been abducted alongside Nobuhiko but managed to escape.
Like Kamen Rider Black, Shadow Moon utilizes the King Stone to transform and is armed with the Satan Saber, a sword crafted specifically to be used by the Century Kings. When he first fought directly against Kohtaro and began losing, Shadow Moon temporarily turned back into Nobuhiko (due to the Creation King's interference) which sadly lasted for a few seconds before said procedure was reversed. He then attacked Kohtaro in cold blood and defeated him. As he seemingly hesitated to land the finishing blow to Kohtaro, he tightly gripped his Satan Saber and killed him. He was then ordered by the Creation King to take Kohtaro's King Stone and was slowly about to when Kohtaro turned back into his human form. The human form of Kohtaro seemingly caused Nobuhiko's humanity to slightly reawaken as he said Kohtaro's name once more and caused him to not only hesitate but also decide against taking Kohtaro's King Stone. As he proclaimed that he didn't require Kohtaro's King Stone to become the next Creation King, he was then questioned by the Creation King who asked if he regained his humanity upon temporarily reverting back to human form. Despite denying these accusations, Shadow Moon himself stated that he couldn't do it before disappearing away.
After having a rematch with a resurrected Kohtaro, he was weakened by Kamen Rider Black using the Satan Saber to slash the Kingstone. After being defeated, Shadow Moon then harshly told Kohtaro that he shall feel regret for the rest of his life knowing that he killed his best friend Nobuhiko and stated that he was the next Creation King. Despite saying such a cruel thing to Kohtaro, Shadow Moon was told to stay alive as Kohtaro promised he would save him. His last known words in this series were Black Sun.. Minami Kohtaro". He was left inside (due to the place exploding) the Gorgom Headquarters when his Kingstone exploded.
He is voiced by Sam Neil in English Dub and reprised by Masaki Terasoma in Japanese Dub.