Angel Island is a floating landmass above Sonic's world, kept afloat by the mystical Master Emerald. Both the island and its source of flight is often guarded by the islands sole resident, Knuckles the Echidna.
Knuckles the Echidna has guarded Angel Island for a long time, however, there were times where Knuckles had to leave his post and leave it unguarded for one reason or another. Most recently, during the war between the Eggman Empire and the Resistance, Knuckles left the Island in order to become the commander of the Resistance. (StH: #3)
The Battle for Angel Island
Eventually, Neo Metal Sonic discovered that, with Knuckles on the surface, the island and the Master Emerald was unguarded. He took note that the Master Emerald would help accelerate his plans. (StH: #3)
Ready to proceed with his plans, Neo Metal Sonic invaded Angel Island's airspace using the Egg Fleet and its forces. With no resistance from the unguarded island, Neo Metal Sonic managed to establish his forces there and put its airspace into lockdown with his Egg Fleet. He then secured the Master Emerald for himself inside a fortification he built around the Master Emerald's shrine. (StH: #7, #8)
- Angel Island first appeared in Sonic the Hedgehog 3. As the home of Knuckles, and the location of the important artifact, the Master Emerald, it has since become a reoccurring location for the series.