It might be "Childhood's End" one of my all-time faves!
Childhood's End explores humanity's transformation and integration with an interstellar "hive mind" or Overmind. It also touches upon such matters as cruelty to animals, man's inability to live in a utopian society, and the apocalyptic concept of The Last Man on Earth. The 1953 edition of the story begins at the height of the Cold War, a few decades after the fall of the Third Reich, with attempts by both the United States and the Soviet Union to launch nuclear rockets into space for military purposes, threatening imminent doom for the planet.
The humans' arms race is brought to a halt by the sudden appearance of mysterious spaceships above all the Earth's great cities. After a week of silence and increasing tension, the aliens, who become known as the Overlords, announce by world-wide broadcast that they have benign intentions and desire to help humanity but also that they will henceforth assume the minimum amount of control which will achieve their aims. As enforcers of peace, they bring salvation and life. They also bring the death of some dreams, as humanity is no longer completely independent and may not pursue certain scientific goals, such as space exploration.
The humans remain suspicious, as the Overlords never appear in person. The Overlords' representative, Karellen, does speak with the Secretary General of the United Nations Rikki Stormgren, but is always hidden behind a pane of one way glass. The two develop a great deal of respect for one another, though it is clear they are not equals. To allay the inevitable suspicions of some, Karellen promises the Overlords will reveal themselves physically in fifty years, after humanity has matured and become comfortable with their presence.
Under the (mild) domination of the Overlords, Mankind enters a golden age of the greatest peace and prosperity ever known, albeit at the expense of some creativity and freedom. Not every Earthling is content with the bargain, nor accepts the beneficence of the Overlords' long-term intentions. Stormgren, with Karellen's help, survives a kidnap attempt by subversive humans suspicious of the Overlords. Stormgren secretly harbors lingering curiosity about the real Overlord nature and smuggles a device aboard Karellen's spaceship to see behind the one-way screen that separates them. Years later he tells a questioning reporter the device failed. The novel strongly hints that the device did indeed capture an image of the Overlords, which Stormgren saw, but that Stormgren agrees with the Overlords: mankind is unready for what that image revealed.
True to their word, fifty years after arrival, the Overlords appear in person. They resemble the traditional human folklore image of demons: bipeds with large wings, horned heads, and tails. The Overlords are taller than humans and of proportionally more massive bodies covered with a hard, black armor shell. The Overlords' resemblance to the devil of human folklore is explained as a form of racial memory and it is assumed (and not contradicted by the Overlords) that they had visited the Earth many thousands of years previously. The presumed failure of the experiment at that time led to the memory. The light from Earth's sun is too harsh for them, because their planet's sun has a dimmer redder light, and, though they can breathe Earth's atmosphere, the mix of gases in their own atmosphere is more comfortable. Humankind has, however, grown accustomed to the Overlords by this time and accepts them with open arms, and with their help, creates a Utopian world.
Although humanity and the Overlords have developed peaceful and even friendly relations by now, the spread of equal goods and the ban on building space ships capable of traveling past the Earth's moon causes some sects to believe their innovation and independence is being suppressed and that culture is becoming stagnant. In response, those sects establish "New Athens", an island colony.
Some ten years after the Overlords revealed themselves to humanity, human children (starting in New Athens) begin displaying telepathic and telekinetic abilities and as a result, become estranged from their parents. Karellen then reveals the true purpose of why the Overlords came to Earth. They are in service to the Overmind, a cosmic mind amalgamated from ancient galactic civilizations, freed from the limitations of ordinary matter. The Overlords are not themselves capable of joining the Overmind, but the Overmind has charged them with the duty of fostering humanity's transition to a higher plane of existence and merger with the Overmind. The racial memory of the devil in human folklore is now revealed not as a memory of past events but as a prescient "memory" of the Overlords role in the extinction of Homo sapiens that results from the childrens' evolution. Karellen expresses an envy of humanity; his race is trapped as they are, as they are not now capable of joining the Overmind, though he hopes they will eventually learn how to do so.
Karellen announces that the children with psychic powers will be segregated from the rest of humanity on a continent of their own, and only these children will merge with the Overmind. No more children are born; the narration subtly hints that most of the parents commit suicide, while their children evolve towards merging with the Overmind. New Athens is then destroyed by the leaders, by detonating a nuclear bomb.
The last man alive is Jan Rodricks, a physicist, who will witness mankind's final evolutionary transformation. He stowed away on an Overlord supply ship earlier in the story in a successful attempt to travel to the Overlord home planet, which he correctly guessed orbits a star of the Carina constellation. As a physicist, Rodricks knows of the relativistic twin paradox effect: the Overlords' ships travel at a significant fraction of the speed of light, and as a result, the trip to the Overlord planet and back to Earth will only take four months in his subjective, personal time-frame, but the amount of elapsed 'objective' time will be, at minimum, 80 years, or the length of time light would take to make the similar journey, although the actual trip takes much longer. (The Overlord star system " known as NGS 549672 to astronomers on Earth " is forty light-years distant from Earth.)
When Rodricks returns from the Overlord home world, he expects no one on Earth will remember him, nevertheless, he is unprepared for the return: mankind, as he knew it, died. About three hundred million naked young beings, physically human but otherwise with nothing common to Man, remain on the quarantined continent. They are the final, physical form of human evolution before merging with the Overmind. Life on their continent " not only human life, but all other forms also " was exterminated by them, and the vast cities that Jan remembers are all dark.
Some Overlords remain on Earth, studying the evolved children. It also is revealed here that the Overlords have met and conditioned other races for the Overmind, and that humanity is the fifth race whose apotheosis they have witnessed.
When the evolved children have grown strong enough to mentally alter the Earth's rotation and affect other planetary adjustments, it becomes too dangerous to remain and the Overlords prepare to leave. They offer Rodricks the opportunity of leaving with them, but he chooses to remain as witness to Earth's dissolution; mankind's offspring evolved to a higher existence, requiring neither a body nor a place, so ends mankind's childhood.
The story's last scene details Karellen's final backward look at the Solar System, which becomes no more noticeable among the stars as it recedes than the loss of one small planet in the system. He is emotionally depressed, having seen yet another race evolve to the beyond, while he and his race must remain behind, limited to their current form. Despite that, he renders a final salute to mankind, considering whether or not conditioning them for the Overmind helped his goal of deciphering the evolutionary secret for his race to merge with the Overmind. He then turns away from the view, the reader presumes, to await the Overmind's next order.