Takayuki Tsubaki

Ten thousand years ago, a massive battle known as the Battle Royal was fought by fifty-two Undead, each representing a species fighting for dominance over all others. The winner was the Human Undead, known as Category Two of Hearts, giving dominion of the Earth to humanity. In the present day, archaeologists discover the sealed Undead, and accidentally set them free. Thus, a new Battle Royal begins. (mydramalist.com)

Emu Hojo awakens in a mysterious place, with no memory of how he got there. He meets other Kamen Riders - all of whom remember dying. Fragments of painful knowledge dance in his mind; words repeat and rooms reset. Who is pulling the strings of the dead and the Undead? Has Emu died - and if so, how many times? And what does this have to do with the power of the ultimate team, Kamen Sentai Gorider? (Ununnilium)

Four years after an alternate ending to the series, the Undead has been re-released and Glaive reveals himself to be the Albino Joker. The four Riders are again united, intent on saving Amane and stopping the Albino Joker.