Thalarion is an island 300 nautical miles from Themyscira. It was raised by Zeus from the sea floor to be a home for his champions, the Olympians, who would from then on be also called Thalarions.
The only notable features on the island are a city made of crystal, so that the Thalarions would have no secrets from each other, and a river made of gold, so they would never have wants and so they would never succumb to greed. Both the river and the city were made by Zeus.
Achilles is the current king of Thalarion, as well as the king of Themyscira.