The Avatar Isles (Avatar Islands) are a smallish continent formerly belonging to the Avatar, consisting of continental islands of various sizes, akin to Great Britain. In its original state it was filled with forests, meadows and generally considered a paradise on earth, as it was given to him and blessed by the Light Gods themselves.
Now, however, it can be more likened to an outpost of hell, with sulfurous wastelands, black rock formations, rivers of lava and foul air. On its shore yellow waves roll, breaking on needle-like reefs, as devoid of any life as the continent itself. The clouds above are the colour of venomous yellow and rust-red, with great amounts of dust in the air, reducing sight.
When Avatar Amadeus was defeated, Keeper Mukrezar relished his victory and ruled over the isles for a short time, ravishing the land and stripping it of all its natural resources before he himself was overturned. In the following interregnum Zarekos proved successful and ruled for two decades over the land, inadvertently poisoning the landscape with the corruption of his dungeon hearts and killing the inhabitants that hadn't managed to flee, until he was in turn removed in a violent but relatively short struggle. Sailor Mercury, the winner, was granted official ownership over the Avatar Isles after some negotiation. The recognition by the Light Gods provided her with an automatically expanding claim on the yet unclaimed parts of the land.
The change of ownership of the islands caused the corruption generated weather to change as well, replacing the previous yellow and rust-red with a roiling rain heavy storm. This clash of phenomena was most prevalent as Sailor Mercury's claim expanded into Zarekos's territory, causing a torrential downpour where the territory and their corresponding corruption caused weather met. Effects on the shores are unmentioned.
At the present [As of chapter 175 ], the vast majority of the islands remains claimed by Sailor Mercury, despite her not possessing a dungeon heart on the islands themselves anymore. Likely the area remains claimed to Sailor Mercury due to her claim as empress.