Before the birth of humanity, on another planet, the race now known as the M'krann developed individuals who could be referred as mutants. That species of cosmic mutants then evolved a step further, becoming the Celestials.
One of those, Xodus, the Harvester, knew that mutants would spawn on other worlds and that some would eventually rise to replace his kind. He consequently decided to destroy the mutants wherever they were to be found.
His brethren, seemingly led by Arishem, found those actions of indiscriminate destruction to be unscientific, and banished him to a dimension beyond their universe, making him Xodus, the Forgotten.