Tiamut was a Celestial growing within the Earth from a Celestial seed planted millions of years ago, feeding off the energy from the planet's sentient lifeforms. When a population quota of intelligent beings on Earth was reached, Tiamut would hatch from the planet, resulting in its destruction and all life on it. The growth and evolution of Earth's inhabitants was threatened by the Deviants, a race created by the Celestials and sent to planets that incubated Celestials in order to suppress apex predators. However, the Deviants went rogue and became apex predators themselves. In order to destroy the Deviants, the Celestials created the Eternals, a group of which was sent to Earth to protect its population and thus ensure its growth. Out of these Eternals, only the Prime Eternal Ajak and Ikaris knew that the ulterior motive of their kin's mission to protect the Earth was to ensure Tiamut's birth.
In the modern age and shortly before Tiamut's emergence, Ajak contemplated stopping the process, having grown fond of humanity. Ikaris murdered her to prevent her interference. The rest of the Eternals learned the truth after Sersi inherited Ajak's ability to communicate with Arishem and the Celestial explained their true purpose to her. Sersi rallied the rest of the Eternals to stop the emergence. The Eternals initially planned to use the Uni-Mind designed by Phastos to augment and funnel their cosmic powers into Druig so that he could put Tiamut to sleep. After coming into conflict with Ikaris, the rest of the Eternals used the Uni-Mind to empower Sersi, and she used her transmutation powers to turn Tiamut's hatching body into marble, putting a stop to its emergence, and leaving only the creature's head and hand above the surface.
- Tiamut has a sign name in ASL, which is an index finger bending as it moves away from the temple.