Referred and given different names by different people throughout history, the entities commonly known as the Celestials are revered in legends as beings capable of breathing new life into galaxies and giving birth to planets themselves.
In Marvel Comics, the Celestials influenced key events in human history for mysterious and unclear reasons. They were responsible for key human evolutionary events, such as the genetic-offshoot races the Eternals and the Deviants as well as the emergence of super-humans. They were also credited with influencing and aiding the development of some ancient advanced civilizations such as the Aztecs.
The Celestials' agenda is opposed by the Watchers. Due to having sworn an oath of non-interference in species younger than themselves, the Watchers found the Celestials' genetic engineering of such species to be the antithesis of what they believed. Thus, the Watchers and the Celestials were in conflict for billions of years.