There are multiple supreme beings in the Marvel Universe that have helped shape the very fabric of its reality. Some are benevolent and seek to guide fledgling life and help usher in new ages of peace and prosperity, while others are embodiments of ancient evils that want only the total destruction and subjugation of all of reality. One of the most prominent of these forces is the Celestials, a race of cosmic entities that predate the creation of the universe itself. But despite their immense power, the Celestials can still be defeated, and they have been by some of Marvel's most dangerous villains.
Created by Jack Kirby in Eternals #1, the Celestials are remnants of a time before time, emerging from the space before existence. This was known as the First Firmament and it was the physical manifestation of existence itself. The First Firmament created two races: the Celestials and the Aspirants. When the Celestials rebelled against the First Firmament they initiated a celestial war against the Aspirants and set in motion the events that formed the multiverse itself. As the Celestials have survived through time, they have become akin to forces of cosmic nature who have created life and subsequently destroyed it. Races such as the Eternals and the Deviants were created by the Celestials and even the mutations of homo sapiens can be traced back to the Celestials, who were worshipped and feared for their grand deeds.
The Celestials conduct what are known as Celestial Hosts, events in which they approach planets with intelligent life and judge whether they are deserving of continued existence. But for all of their immeasurable power the Celestials are not without weakness and have even been defeated more than once.
In the original Infinity Gauntlet by Jim Starlin, Ron Lim, and George Perez, Thanos had acquired the Infinity Gauntlet and all six of the Infinity Gems. As Marvel's most powerful cosmic forces realized what a terrible threat Thanos posed, they attacked him en masse; among them were two of the Celestials. But the sheer power of the completed Infinity Gauntlet was simply too much even for the combined forces of the universe’s progenitors. And with this action, Thanos was able to overpower two Celestials.
In the far reaches of space, there exists a place known as Knowhere, a giant decapitated head that serves as both a spaceport and base for various intergalactic groups. It has since been revealed that Knowhere is in fact the severed head of an ancient Celestial. The being known as the Knull, the creator and god of the symbiotes, wielded a sword known as All-Black to kill the Celestial.
Another instance of a Celestial falling in battle came from the most surprising team of all: the Stone Age Avengers. In Marvel Legacy, by Jason Aaron, Esad Ribic, Steve McNiven and Mathew Wilson, it was revealed that a prehistoric team consisting of Odin, the Phoenix Force, Ghost Rider, Starbrand, Agamotto, Black Panther and Iron Fist united to defeat a Celestial known as Zgreb the Sorrower and prevent it from destroying the entire earth.
In the alternate timeline of Earth X by Jim Krueger and John Paul Leon, the Celestials were defeated in a truly fantastic manner. When the Celestials approached Earth Galactus was summoned to help defend the planet. Galactus defeated the Celestials, and upon removing his helmet, revealed himself to Franklin Richards, son of Reed and Sue Richards. Franklin had convinced himself he was Galactus in order to become the Devourer of Worlds so that he might feed off of planets that contained Celestial embryos.
The Celestials are among the most powerful beings to exist within the Marvel Universe, but even they can be stopped. While the Celestials may pose an existential threat to the beings they want to kill, those aims have largely served as opportunities for Marvel's heroes and villains to show just how powerful they really are.