Originally turned by the greater demonoid Valak to become a lesser demonoid, the woman that will later be known as Seir was reborn into a feral greater demonoid instead. The berserking new greater demonoid then turned against her creator and killed it, before slaughtering the rest of the lesser demons in the area, but leaving the humans alive.

In a turn of event, someone managed to subdue the newly born greater demonoid and even suppressed her demonoid cell, reverting the woman back to her human form. Introducing himself as the fabled 'Kamen Rider', the only person rumoured to be able to square up against a greater demonoid, the man explained to the woman that his assistant has crafted a belt that is both capable of suppressing and harnessing the power of the demonoid sleeping within her body.

The man promised to show the rope so that the woman could use her newly found power properly. In the process, the woman learned that the man was in fact, an otherworlder who came to this world for a specific task. After learning that the greater demonoids named themselves with the series of names that bear uncanny resemblance to the names mentioned in an old tale from his home world, the man named the woman 'Seir', citing that the Seir from the old tale was depicted as a beautiful horse rider; fitting as she'll become the rumoured 'kamen rider', replacing him as his main task in this world has now come to an end.

Thus begin the journey of Kamen Rider Seir to rid her world from the menace of the demonoids.

Author's Note:
Some scenes with gore and violence. R-18 contents in chapters with asterisk (*).
I write during weekend (if I don't have any other task at hand) and will post the chapter soon after I'm done. It might take a while, but I WILL update this.

Cover image was created by yours truly by using Adobe Photoshop and several pictures from internet.

Kamen Rider A-Cross, Assistant, Professor, Z-Hero, and D-Evil are OCs created by u/Shiyonichi from Reddit.
Kamen Rider Seir is an OC by yours truly.
Other Kamen Rider(s) mentioned in the story, or when A-Cross/Z-Hero/D-Evil use their power(s), belong to their respective owner.