The Hamidon is the end result of a man transformed into a god in the Praetorian Universe.
Details are vague about its origins, but what is known is that the being was once Dr. Hamidon Pasalima, a scientist fixated on finding what it believed would be "the ultimate answer" about the secrets of the universe.
It is said that the nuclear exchange that devastated the planet during the Korean War compelled Dr. Hamidon to seek out that "ultimate answer". Upon his ascension into a god-like being, "The Hamidon" decreed man to be guilty of destroying the natural balance of the planet, and thus sought to destroy humanity.
The first act the Hamidon did was to give sentience to give sentience to insects, trees, rocks, fungus, and crystal, transforming them into the Devouring Earth. These creatures then began destroying the civilized world, one city at a time, one country at a time.
The world of man would eventually fight back with its ultimate weapons, but even that proved to be useless against the Hamidon. Worse, the prolonged use of these devices actually caused the Hamidon... to change.
Eventually the Hamidon became large enough to conquer much of the world and to spread its influence further through "seeds", floating creatures the size of great whales that carry its own smaller "aspects".
It has been a long time since any human being - powered or non-powers - has seen the Hamidon itself. Scientists speculate that the Hamidon has evolved into a massive Lovecraftian creature that send out smaller aspects to grow and expand. This has been reinforced with the sightings of large tentacle appendages in the remains of several cities as well as the ruins of First Ward.
In "Tales from Paragon City" Issue #2, the story of the Hamidon and the Devouring Earth destroying civilized society is re-told as a biblical tale known as "The Book of the Praetor". In this re-telling of the story, the Hamidon and the Devouring Earth are described as servants to "The Gods of Old".
(Please see its Paragon Wiki entry for more information.)