The creature known as the Hamidon was once a zealous scientist named Hamidon Pasalima. Pasalima believed that mankind was a parasitic infestation that was destroying the natural harmony of the planet. He wanted to give "the Earth" a fighting chance, so he employed every means at his disposal, including the use of dark magics, to empower Gaia, the mystical spirit of the planet. But instead of empowering "the Earth", he ended up using it on himself, transforming himself into a massive being resembling a giant amoeba.
Dr. Hamidon Pasalima was no more. There was, instead, "The Hamidon".
His transformation allowed him to empower inanimate objects, rocks, trees, and crystals, and transform them into his minions, otherwise known as the Devouring Earth. Those minions would then transform humans into mutated creatures called the "Devoured", which he would then use as weapons to destroy humanity.
The Hamidon is a formidable threat to humanity itself. If left unchecked, the Hamidon's Devouring Earth could obliterate humanity and replace it with the "Devoured".
Worse yet, it is believed that the Hamidon is not through "evolving". That the amoeba-like form it is currently in is only the first stage into becoming a truly global creature. A true "Gaia".
(See its Paragon Wiki entry for more information.)
There is a Praetorian version of the Hamidon, which transformed at an earlier stage due to the divergent history of that universe, and thus became more powerful, more destructive, and has evolved even further.