Phantos is a moon of the planet Eternia, governed by a monarchial regime, ruled by Queen Elmora. This world is the main source of Photanium. Phantos, being a moon of Eternia, presumably falls under the nominal authority of the planetary government of Eternia, although the later episode "No Job Too Small" establishes that Phantos is in fact a planet. So it is possible that the moon seen in Phantos' introductory episode, "She-Demon of Phantos" is in fact a moon of the planet Phantos rather than one of Eternia's moons. Elmora and previous monarchs of Eternia have close ties with the ruling royal family on Eternia, as Man-at-Arms in the episode "She-Demon of Phantos" refers to the alliance between Eternia and Phantos.
While it is unclear if Queen Elmora rules over the planet Phantos or merely one of its moons, Phantos has no connection to Eternia's twin moons- the Bright Moon and the Dark Moon- which are seen in the episode "The Shadow of Skeletor".