That's the Jyamato Grand Prix logo. Beroba's death game is starting up again.
On an interesting note, this can be a way for the author to bring in the old man's friend Yume since they did a 'what if' scenario where Yume was a DGP player.
While not on the scale of Grand Orders or Lost Belts, those False Sanctums can twist reality, turning urban legends into reality for example. In the context of this manga, due to the presence of Decade, they instead bring back past villains of Kamen Riders.