Kamen Rider Ryuki: Into the Abyss

The setting takes place a few years after the events of Kamen Rider Ryuki. (specifically where the events of Kamen Rider Blade is still going)

Everyone who was involved with the Rider War are living peaceful lives. However, it all changed when Mirror Monsters randomly appear out of nowhere. Where they start to cause mysterious disappearing incidents throughout the city.

The protagonist Kiyo Amamiya, is a young man who is mentally ill after the death of his lover. But his road to recovery was cut off when he encountered the Mirror Monsters. Kiyo found out the truth that his lover is alive and a part of the disappearances.

Kiyo also found out a terrible revelation about the remains of Kamen Rider Odin, who's responsible for the Mirror Monsters. Now Kiyo must take up the mantle of Kamen Rider Abyss to venture out to the Mirror World, defeat all the Mirror Monsters, save his lover, and destroy Odin's remains before another Rider War begins.

This story explains about what actually happened to the 14th rider in the Main Timeline Continuity.