The planet Eternia only briefly appears in The New Adventures of He-Man, but is often referenced, most noticably when Adam of Grayskull holds up his sword and says "By the Power of Eternia". Further more, the Starship Eternia is named after the planet.
History[]
When Captain Hydron and Lieutenant Flipshot used a Time Vessel to travel from the future to the planet Eternia, they were soon captured by Andros, a minion of Steletor, and taken to Snake Mountain. Skeletor became intrigued by the time travelers and released them from his dungeon, pretending to be on the side of good.
Meanwhile, the Sorceress, informed He-Man that it was his destiny to travel to the future and continue serving as the defender of good with new companions. As Prince Adam, Eternia's greatest hero went to the Royal Palace to bid farewell to his parents, the King and Queen. He also revealed his secret identity to them at last.
Shortly afterwards, both Skeletor and He-Man ended up accompanying Hydron and Flipshot on the journey back to their own time. The Sorceress predicted that Eternia would have prosperity and joy for all time without Skeletor. This was He-Man's legacy. Indeed, soon after Skeletor departed, Snake Mountain began to decay and whiter, while the land surrounding it prospered.