- "I call it the "Expansion". It is my purpose, and now it is yours as well. Over thousands of years I implanted thousands of extensions of myself on thousands of worlds. I need to fulfill life's one true purpose, to grow and spread, covering all that exists until everything is... me!"
- ―Ego to Star-Lord
The Celestial Ego became convinced that he was the only Celestial remaining in the cosmos while presuming his race had become an endangered species that was on the verge of extinction. Born bodyless and parentless shortly after the Universe's creation, Ego's origin remains mysterious and unknown by nature. In fact, according to Ego, he was born as a large brain-like resemblance that journeyed among the stars while, at the same time, gained sentience and sapience, even though he dwelled in a state of extreme loneliness. As time passed and Ego continued to develop, he soon constructed a shell to protect himself and then afterwards, built an equivalent planet-shaped structure to further enhance his self-protection. Throughout countless eons, Ego remained in solitude while consciously watching galactic civilizations rising and falling simultaneously, regardless of his living in immortality. Yet after bearing witness to death and decay among life in the cosmos, Ego eventually realized that all life other than Ego's, due to being an member of the nearly extinct Celestials, was generally futile due to its ephemeral nature. This caused Ego to find what he thought was the actual meaning of life: to assimilate all life in the cosmos and conquer it for himself.
Throughout his many attempts to accomplish this, Ego traveled among the stars to arrive on thousands of life-sustaining planets and implanted thousands of extraterrestrial seedlings (extensions of himself) on all the planets he visited. Once the seedlings were all activated, massive blobs would burst from the planets' soil and initially consume all planets that sustained life at once. However this strategy required an immense amount of Celestial energy to unleash the activation which made it impossible since Ego was not able to locate the presence of any other Celestials; however, he decided to take the most direct course of action via the creation of a new race of Celestial hybrids by impregnating thousands of female members from multiple extraterrestrial species that were located in every life-sustaining planet that could be found within the cosmos, of which Ego then hired Yondu Udonta and the Ravagers to deliver his progeny. The overwhelming majority of Ego's biological offspring, however, failed to inherit the requisite Celestial DNA for the expansion, which resulted in Ego killing thousands of his own children, until Ego's biological son, Peter Quill, came along as Ego's first child to inherit the Celestial genes. Accompanied by the Guardians of the Galaxy, Quill settled on Ego's planet and was soon deceived by Ego in an effort to drain Quill's Celestial DNA. Ego then attempted to absorb Quill's Celestial energy into his own brain, subsequently beginning Ego's terraforming expansion in the process.
The Guardians of the Galaxy fought against Ego to prevent Ego from completing his expansion within the universe. To kill Ego, and considering that there were no other options to kill him, the Guardians of the Galaxy had to travel further underground to find Ego's brain and destroy it. While Ego fought Peter Quill, who had at that stage reached a matching god-like status, Groot planted a nuclear bomb on the surface of the brain. Meanwhile, with every chance Ego got to absorb Quill's Celestial power, the planted seedlings expanded dramatically, consuming entire towns and all their constituent lifeforms in the process. Yet the expansion was short-lasting, as Peter Quill stalled his father enough so that the bomb which was placed on Ego's brain detonated, thus killing Ego. As a result, all of the expanding blobs within the cosmos itself ceased when Ego died.
When the bomb exploded and Ego died, the planet soon collapsed. Yondu Udonta gave up his spacesuit to Peter Quill in order to save him, while sacrificing himself in the process as he dies within the oxygen-less atmosphere of space. Nebula reconciles with Gamora, but chooses to leave anyway, and the two part as caring sisters. The Guardians give Yondu a traditional Ravager funeral, and dozens of Ravagers, including Stakar Ogord, arrive to pay their respects, releasing a string of fireworks.
- In the comics, Ego attempted to assimilate and make himself part of the planet Earth in various occasions.