Coming from the world of Kamen Rider Ryuki, he never actually appeared in the Ryuki series. He was an original Rider that appeared in Episode 6 of Kamen Rider Decade.
If you don't know about Kamen Rider Decade, well, it's a bit convoluted to explain. But in simple terms, Decade is a Rider who travels to all the other Riders' universes (call AR World). Because of this, the series feature many (most) of the other Riders that came before him.
Kamen Rider Abyss also appeared (briefly) in the "Let's Go Kamen Riders" film.
This is a strange and interesting choice for Bandai and Figuarts, but it's proof to their commitment to create figures from every Rider in the show's nearly 50 year history.
He's set for a September release. The price is still TBA.