The First Ones are an alien race of intergalactic explorers native to Eternia. They fought a protracted war with Horde Prime in which he claimed victory, though little is known about the conflict or their current condition.
The First Ones were a seemingly family-oriented peaceful group of humanoids with control over powerful tech that could send holographic images to outer space.
The First Ones were great explorers that once explored the planet Etheria, leaving behind powerful tech and ruins scattered all over the planet, and many pieces of it are used as fundamental blocks for most of the kingdom's societies.
They have a unique written language, First Ones' writing, that so far only Adora has been shown to be able to read accurately. This knowledge helped her to discover her She-Ra identity. George and Lance can read the writing with limited accuracy. Some think she got the ability/knowledge to read First Ones' language for the first time when she touched the sword and connected with its power (and got flashes of memories/knowledge about her past).
The First Ones explored diverse planets in the universe, colonizing and extracting magic from them like they did with Krytis.
Centuries ago, the First Ones colonized Etheria to study its natural magic. As part of the Heart of Etheria Project, the First Ones retrofitted the planet with First Ones' tech, siphoning bits of magic from the Runestones and their connected princesses and storing it within the core of the planet, the Heart of Etheria. The Heart was to be unleashed as a weapon of mass destruction against the First Ones' enemies in the apparent war they were fighting. When activated, this concentration of power would forcibly accumulate within She-Ra, who would fire the weapon. In an effort to control the weapon, the First Ones created the Sword of Protection in order to manage the She-Ra that wielded it, and also created Light Hope, an interactive A.I. program, to be Etheria's facilitator as well as She-Ra's trainer and guide. One of their operatives, Mara, was chosen by Etheria to be the next She-Ra and was given the sword by the First Ones. She was sent to study Etheria's magic and was kept mostly in the dark about the truth of the Heart of Etheria Project.
When Mara discovered the truth about the project and that Light Hope had been reprogrammed by her superiors to activate the weapon at all costs, Mara sought to stop it as the activation threatened to tear the entire planet apart. The First Ones then abandoned Etheria for this reason, as they were unable to control the weapon as they desired. To keep the weapon from being used on anyone else, Mara used the last of her strength to open a portal and send the planet into Despondos, where it could remain hidden and keep the universe safe from the Heart's destruction.
However, they were engaged in a long war against The Horde, a war that ultimately toppled their empire and wiped most of them out.
Sometime later, Light Hope was able to open a portal through which she brought to Etheria a baby First One, Adora, so that she someday would become the new She-Ra.
According to Adora, the First Ones are gone and their war long over, but Light Hope, as she tries to activate the Heart, declares the First Ones will rise to glory once more. However, Adora managed to talk her out of it and Adora broke the sword which interrupted the process of the heart.
Horde Prime indirectly mentioned them when he showed Glimmer his room of trophies from previous victories, one of which was a First One miscellaneous. Prime acknowledged the centuries long war he and the artifact’s previous owners engaged in, before informing Glimmer of how he toppled their empire, partially confirming that the First Ones are now few in numbers if not wiped out all together (other than Adora).