Neptune's Kingdom is the realm controlled by Neptune (KQGS) beneath the sea, and lies near to Kolyma and the Enchanted Isles (KQGS).
Kolyma (KQGS) (Eastern Kolyma (KQGS) and Western Kolyma (KQGS)) lies near the sea. The isles are under Kolyma’s jurisdiction. The lands of Avalon (KQGS), and Tanalore (KQGS) are near or border the sea as well located on the continent of Daventry (KQGS) in the Midwestern Hemisphere.
Graham wished to visit the realm on one final adventure before he died.
Behind the scenes[]
The Sea appears to be geographically much different than it appeared in previous series, while its suggested that they exist to the far south which is reasonable for both Kolyma and Enchanted Isles based on released maps, Tanalore is a continent to the northwest of Daventry and not far south, whereas in this series it appears to be a lesser island within the chain. Avalon is less clear, but references to it in original series were limited to begin with.
It's not known if Graham actually visited Neptune's Kingdom in the Reboot Universe previously. In the original universe Graham visited the realm in order to find three keys. However that is irrelevant to the story in reboot universe, as the tower is in Kolyma itself, rather than a separate island.