Onyx Island is the setting for most of the 2006 PSP game Sonic Rivals, containing five out of six of the game's Zones. A mysterious island that "suddenly appears in the south seas as if out of nowhere" (Sonic Rivals EU Manual), the landmass attracts the attentions of Sonic and Tails, who arrive to investigate - only to be confronted by a disguised Eggman Nega, who turns Tails into a card using a strange camera. Knuckles, Shadow, and Silver all find their way to Onyx Island too, on the search for their own objectives.
As Knuckles' story through Sonic Rivals progresses, he puzzles over the fact that parts of Onyx Island seem strangely familiar. It is eventually revealed by Eggman Nega that Onyx Island is in fact the echidna's own Angel Island from an unspecified point in the future, warped back to the present day by Nega's science and the power of the Master Emerald - apparently just as a vehicle to bring Nega into the past, so he can eliminate Dr. Eggman.
At the end of the game, after Nega is defeated aboard the Egg Destroyer, Knuckles resolves to use the Master Emerald to return Onyx Island to its proper time.
Although far removed from the Sonic game's present timeline, Angel / Onyx Island is still the resting place of the Master Emerald in the future, as Nega admits that he had to come to the island to photograph the gem. Trapped in a card, the Emerald would be unable to keep Onyx Island in the air, which is presumably why it turns up in the sea when it is warped to the past.
Zones on Onyx Island include:
The final level of Sonic Rivals takes place high above the island, on Meteor Base Zone, an asteroid-mining complex and shipyard in the deeps of space.
Onyx Island appeared in the Archie comics in issue #215, in the short story Future Tense. The island is shown crashed ito the sea at an angle, and plays host to a destroyed and deserted city which Silver the Hedgehog investigates. He is looking for clues of a traitor to the Freedom Fighters centuries previously, which allegedly caused the devastation on the island. Using clues from the city's library, Silver concludes that Rotor Walrus is the likely culprit, and uses Chronos Control with a Time Stone to travel to the past and eliminate him.