Arretan
The Arretans were a humanoid race native to the planet Arret.

History 
They explored the galaxy and colonized several planets, possibly including Vulcan, about six hundred thousand years before the 23rd century. Around a hundred thousand years later, they achieved such great mental power that they began to regard themselves as gods. War broke out, "a struggle... for such goals, and the unleashing of such power, that you could not comprehend," as Sargon described the event. The war tore the atmosphere from their planet, ultimately extinguishing all life save for a relative handful of representatives. Again according to Sargon, "only the best minds were chosen to survive." 

Those chosen transferred their consciousness into receptacles: globes approximately fifty centimeters in diameter. Sargon's receptacle occupied a special pedestal in the main chamber of a cave constructed 112.37 miles beneath the surface of the planet. From there, he carried out the task of probing the heavens for starships representing a race advanced enough to save his people from oblivion. The remaining dozen or so of his peoples' receptacles occupied niches in a smaller side chamber.

By 2268, only Sargon, his wife Thalassa, and Henoch (a member of the opposing faction) still survived. In that year, the  discovered the spheres containing the aliens' consciousnesses. ()

Powers and abilities
The aliens displayed various abilities:
 Sargon affected the Enterprise instrumentation sufficiently to lure the ship to his world from a great distance.
 At closer range, he could speak telepathically, reading the thoughts of those in orbit and replying in kind. Neither Henoch nor Thalassa demonstrated this power.
 Sargon could, at closer range, neutralize all ship's power (without damage), and enhance the ship's transporters and communications systems to permit transfer of matter and information between the ship and the landing party located more than 112 miles below the surface of his planet.
 Sargon persuaded Ann Mulhall that she had received an order to report for landing party duty. It was only when Kirk asked her from whom she'd received the order that she became puzzled, but even then she remained convinced of the order's legitimacy.
 All of the aliens could transfer their minds to suitable host bodies, and back again. Sargon could briefly house his mind within the machinery of the Enterprise.
 All of the aliens could direct energy at other beings and objects. Thalassa used this power to torture Leonard McCoy for a few seconds before realizing her moral error. Sargon used it to destroy the receptacles that once housed their minds.
 All of them demonstrated great skill in engineering; Henoch's understanding also encompassed pharmacology. It is likely that all of them possessed superior intellect relative to Humans.
 Sargon, or possibly Sargon and Thalassa acting together, placed two consciousnesses within a single body.
 Sargon could hide his presence from others of his kind; he successfully prevented both Thalassa and Henoch from realizing he still lived until he chose to reveal himself. Thalassa did not understand this until Sargon explained that "he had power Henoch does not suspect."
 Henoch could telepathically torture Humans in some invisible fashion. He used this power to "make an example" of Uhura, presumably when she refused to follow his orders.
 Henoch could make multiple Human individuals experience sourceless but intense pain.
 Thalassa also referred to powers "Sargon did not permit them to use" because he believed them too tempting.
 Sargon warned Kirk that while his race [like Humans] had surived their primative atomic age only to tell of an ultimate crisis (that destroyed their world)-that all races must face.

One important limitation seemed to be the requirement for a suitable vessel; these aliens could not exist without either a body or a receptacle of some type. This differentiated them from true noncorporeal life such as the Organians or the Q. In addition, their presence placed great strain upon Human and Vulcan host bodies, requiring frequent injections of a "metabolic reduction agent" to compensate. In time (months or years), host bodies grew accustomed to the aliens' powerful minds, however.

Individuals
Henoch
Sargon
Thalassa

Appendices

Background information 
This species name was not identified on screen, but rather was identified in the script notes in the final draft of "Return to Tomorrow", dated . Furthermore, in two separate lines cut from the same script, Sargon's people were known as the Arretans or Arretians. 

Spock theorizes that the Vulcan species might actually be descendants of colonists of Sargon's species, stating "That would tend to explain certain elements of Vulcan prehistory." T'Pol, however, stated in  that Vulcans evolved on Vulcan. Nevertheless, this was roughly a century before "Return to Tomorrow" took place and T'Pol may have just been citing the scientific theory of her time.  then established the Romulans, and therefore the Vulcans, to have originated from DNA sequences seeded on numerous worlds by the ancient humanoids 4.5 billion years ago who were even older than the Arretans. If Sargon's people resulted from DNA by the ancient humanoids themselves, both "Return to Tomorrow" and "The Chase" are in fact compatible.

Apocrypha
Greg Cox's novel Q-Strike from the  series suggests that the creature known as (*), aka the Beta XII-A entity, was the cause of the war among Sargon's people which destroyed the planet's atmosphere.

Christopher L. Bennett's novels Ex Machina, The Buried Age and Watching the Clock referred to them as the Sargonians, a name given to them by Federation archaeologists. The latter book indicates they had founded the Arret Empire, and terraformed and colonized many worlds across the galaxy for a hundred-thousand years. It also theorized that they were the ancestors of not only the Vulcans, but also the Deltans.

External link

Species
Memory Alpha articles named from scripts‎