Theories for Eternia should be posted here. These are theories or ideas not canon within the show.
A thousand years ago, Eternia lost its battle with the people they were going to use the Heart of Etheria on. They were overrun by their enemy and later that enemy was overrun by Horde Prime. Mara would be considered a traitor by the First Ones and no one connected with her would ever be welcome on Eternia again. W.s
If Dreamworks releases a She-Ra movie or an epilogue-season a few years from now, we may get a closer look on what Eternia looked like, who the ancient First Ones were (including the First Ones leaders the others went against), and who Adora's biological parents were. We could possibly see more of Mara's backstory; how she became She-Ra and the places she originated from, and how others saw her before she sacrificed herself.