However, Ego needed another Celestial's assistance to activate them, so he fathered children with various alien races and had retrieve them so he could gauge their Celestial powers.
As a result of this ability, the Celestials can manipulate matter at the molecular level.
Loki surmises that the Celestials feared succumbing to the Horde themselves and believed it better to leave Zgreb buried deep underground to keep the Horde infection contained, creating in the process two opposing humanoid species — the Eternals and the Deviants - to defend the process under the false impression that they were actually protecting the human race.