the epitaph of twilight

Now your power is at its peak

10,000 years ago, a group of immortal monsters known as the Undead faced off in an intense Battle Royale, each one representing a different species of life on Earth, to determine which species would become the dominant life on Earth. The Human Undead won, sealing all other Undead within playing cards, and thus was our civilization allowed to thrive. Now in the modern day the Undead have been released from their seals. The secret organization BOARD recruits Kenzaki Kazuma to fight as Kamen Rider Blade, fusing with the Category Ace of Spades Undead to use its power against the immortal monsters. However, it is not long before Sakuya Tachibana, who fuses with the Category Ace of Diamonds to become Kamen Rider Garren, betrays the organization. In the wake of that destruction only Kenzaki and Shiori Hirose emerge to continue the fight to defend humanity in the new Battle Royale.

( Detailed but non-spoilery thoughts on the show within. )