During the Second Celestial Host, the Dreaming Celestial was exiled by his brethren.
At the time, the Deviants were the dominant force on Earth, and the Dreaming Celestial was ready to call the Horde for harvesting of the planet. However, Arishem had that order canceled, and had many of the Deviants culled, while not touching the other species. As this was a violation of their protocol, the Dreaming Celestial tried to assume command, and attacked Arishem. The Dreaming Celestial then found himself attacked by the other Celestials, and he was imprisoned underneath the mountains. Why they did this though, is still not truly clear, and mention must be made of the fact that this account of events was given by the Dreaming Celestial itself and has not yet been rebutted or verified.
The other Celestials ripped his spirit from his body and placed in a device known as "The Vial," with a fraction of the Dreaming Celestial's soul used to make a key that could unlock the Vial. The Celestials then sealed the Dreaming Celestial's body in a vault under the Diablo Mountains in California. For millennia, the Celestial slept. Eventually, the Deviant Priestlord Ghaur remade the key and drank the essence of the Dreaming Celestial, thus giving the Priestlord the height and power of a Celestial. Soon thereafter, the Dreaming Celestial took advantage of this and mentally controlled Ghaur to free the Dreamer from his never-ending sleep. This plan was thwarted by the Avengers and the Eternals, and the Vial was sealed once more.
After Franklin Richards transported the Fantastic Four and several Avengersinto a pocket dimension, the Dreaming Celestial found a loophole out of his prison: a gateway into the pocket universe. The Dreaming Celestial learned that one of his star-spawned kin, the Celestial known as Ashema, was the guardian of the pocket universe. He concluded that the only way out of that pocket universe was through her, and began to plot her demise. Soon he was opposed by Doctor Doom, Lancer, Technarx, and several other heroes. The Dreaming Celestial was successful in capturing Ashema and escaping into his home dimension, but was soon thwarted by Doom and the Fantastic Four.
The Deviants eventually re-awoke the Dreaming Celestial, who turned from black to gold when the rising sun's rays hit him. However, for reasons as yet unrevealed, the Dreaming Celestial decided against destroying the Earth as he had originally intended, at least for the moment. The Dreaming Celestial decided to wait and judge humanity for himself by his unknown standards. In the interim, he informed Makkari that a devastating force called The Horde, drawn to the planet by the Celestial's awakening, is heading for the Earth.
The Dreaming Celestial still silently stands in place, towering above San Francisco where he was ultimately turned into a tourist attraction. Although the Dreaming Celestial lays apparently dormant, he speaks to, and once even spoke through, Makkari, his prophet. On that occasion, he referred to himself as Tiamut the Communicator.
He aided Makkari and the Eternals in finding their brothers and sisters that were stuck in their human guise, but when Makkari was killed he initiated a fail-safe, the Song of the Sleeper, which placed all humans in an unconscious state until the world would be reset to its pre-inhabited state, destroying all life. He was only willing to end the song when Sersi sacrificed her life to allow Makkari to be reborn in her body.
After The Horde arrived on Earth, the Dreaming Celestial finally confronted the Fulcrum, questioning his own purpose. In the process, he removed his helmet, revealing a shining humanoid face. He then pleaded with the Fulcrum to spare Earth as the Celestial was finally caring for the planet. The Fulcrum declared that it had sought a Celestial who would rise above its purpose since before time began, and asked the Dreamer to become his companion; he accepted.
After creating a diversion by having Magneto attack San Francisco, the High Evolutionary removed a specific item from the Dreaming Celestial's head through a process Magneto refers to as a "lobotomy". The object is used in an experiment on Magneto which appears to restore his mutant gene.
When Archangel sought to bring about the Age of Archangel, he traveled to the Dreaming Celestial to retrieve a Death Seed. Like Apocalypse before him, Archangel desired to serve evolution and need his horseman Death by his side.
After the schism between the X-Men, Mr. Sinister returned, and somehow activated the Dreaming Celestial. While the X-Men battled the Celestial's body, Sinister utilized its head to transform part of San Francisco in his own image.
Cosmic energy control with an unknown upper limit. Armour able to withstand strikes from nuclear warheads. Tiamut seems to be the most powerful Celestial as Galactus feared it and Uatu the Watcher could not see anything when it awoke.
In an alternate future, the Dreaming Celestial awoke and turned the world-devourer Galactus into a weapon that would consume the entire universe, leaving the Dreamer to become the nucleus and founder of a new, darker reality. The Fantastic Four, Thor, Iron Man and Gladiator of the Shi'ar thwarted his plans, resulting in Galactus consuming the Renegade.