In Tiga's Prehistoric Era, I Snatched Camearra!

[As always, I write wherever my dreams take me. If you readers think something’s off, then you’re right.]

Modern-day young man Yun Ye scrimps and saves to buy a Kamen Rider Decade Driver toy, never expecting that inserting a card would blast him into the super-ancient city of R'lyeh, thirty million years in the past.

On the brink of death, Yun Ye activates an otherworldly version of the Decade Driver and transforms into Decade.

From then on, he scavenges corpses on the fringes of battles between giants, turns the tables on lesser monsters, subdues scavenger gangs, and enters the black market, gradually uncovering the darkest truths of the super-ancient civilization:

Humanity is creating gods, the temples are hiding the truth, and the Kirielodians are secretly pushing forward with experiments to create artificial giants. The Giants of Light and Darkness are on the verge of tearing the world apart.

He infiltrates the Temple of Darkness and, through one conspiracy and battle after another, transforms from a Kamen Rider into a Giant of Darkness.

When Dark Tiga finally sees his ideals shattered and betrays the faction of darkness, Yun Ye joins the dark side to replace the king.

With Earth’s affairs settled, the protagonist embarks on an interstellar journey, facing the Land of Light, Belial, the dark forces of the universe, the powers of light and darkness, the Four Mysterious Ultras, Cosmic Phantom Beast Fist, the far side of the universe, antimatter, and a host of other adventures.

He builds the Dark Radiance Kingdom, more powerful than the Land of Light, becomes Belial’s close friend and a son favored by both light and darkness, and ultimately is crowned King of the Universe.

After endless ages, just as he becomes the one and only existence,

he senses another spacetime and descends into the world of Kamen Rider Zi-O. Just as Decade is about to vanish, he saves him and blows up Ohma Zi-O with a single punch.