The First Ones are characters from the 1985 animated series She-Ra: Princess of Power.
The First Ones were the semi-mythical people who either created the planet Etheria or were the first to colonize the world. After a while, the First Ones withdrew to the core of the planet after vesting political authority in a new monarchial, planetary government of Etheria. It is believed the First Ones were actually Ancient Ones from the planet Eternia.
She-Ra: Princess of Power
The First Ones are the Living Spirits of the founders of Etheria. When the stone in She-Ra's weapon, The Sword of Protection, was cracked in battle, forcibly transforming her back to Adora and causing her to lose She-Ra's powers. Light Hope told Adora that only the the First Ones could restore the powers of She-Ra.
At the time of the Great Rebellion, the spirits of the First Ones resided in the Cavern of Fire, which was part of their underground kingdom. To get there, Adora had to overcome many obstacles. She began her quest by entering the Jaws of Darkness inside the Crystal Castle.
When Adora finally managed to reach the Cavern of Fire, her trials had proven her to possess the qualities of She-Ra.
The First Ones told her, Welcome, Adora. We are the First Ones. You have come far to seek our help in returning you to She-Ra. You were brave to climb Skydancer. You were wise to use the shield to outrun the spiked ball. You were swift in escaping the cave-in, agile in avoiding the falling rocks of the Forbidden Corner and brave to risk entering the Cavern of Fire. And you did all this for a friend. Are these not the qualities of She-Ra? Try your sword, Adora.
Adora held aloft her magic sword, finding its up-to-then cracked stone to have been repaired and restored to wholeness, and herself again able to transform into She-Ra, Princess of Power.
The First Ones living in the Cavern of Fire appeared to be made of fire themselves.