Onyx Island is the main setting of Sonic Rivals.
During the game, it mysteriously appears in the ocean, and various characters travel to the island to investigate. On the island, Tails is turned into a card by Dr. Eggman Nega (who is disguised as Dr. Eggman), who uses a special camera.
It is eventually revealed that Onyx Island is a future version of Angel Island that was warped back in time by Eggman Nega with the power of the Master Emerald. Nega has travelled into the past in order to change his destiny by turning the whole planet into a card that he can control.
Even in the future, Nega mentions that Onyx Island is still the home of the Master Emerald, as he needed to travel to the island to photograph the gem and turn it into a card.
Following Nega's eventual defeat, Knuckles decides to return Onyx Island to the future by using the Master Emerald.
Five out of Sonic Rivals's six zones are located on Onyx Island, as follows:
The game's final zone, Meteor Base Zone, takes place in space and not at Onyx Island.
In other media
Onyx Island appears in StH #215 as an island that stands in the sea in a slanted angle. It has a ruined city with a library where Silver researches. It holds many artworks, including a picture of the Future Freedom Fighters and a huge statue that looks like Lara-Su. The island has been mentioned in one of Ian Flynn's question-and-answer sessions, in which Silver mentions that he knows the guardian of Onyx Island in his time.