From ROI Wiki

The Margog is a dark and malevolent creatures, the leader of the Horde and a shapeshifting android. He sees himself as a king and desires to be the ruler of all of Braisail. However, in reality though, he is merely a puppet of Negago, who has employed him to exterminate all life on the planet in exchange for the promise of unlimited power, of which Negago has no plans of actually giving him. Originally a creation of Dr. Slao, he becomes the primary adversary of the Burgon Lions in Lion of the Earth, and proves to be an incredibly difficult enemy to defeat. The only thing that holds him back is a lie, created and circulated by Dr. Slao, which states that, while he is indeed powerful, he is not complete, and thus not as powerful as he can potentially be. In order to achieve this full power, he must capture The Four Children of Dr. Slao and co-opt their power as his own. Slao did this as a means to reign in the Margog and hold him back until he could find a way to defeat him.

The Margog is ultimately defeated. But it requires the combined forces of the remaining Guardians, as well as Ter'ik's hunters and an expeditionary force of the Burgon Lions. His two right hand men are the demons Segarg and Regnig, direct servants of Negago sent in among the ranks of the Margog to keep tabs on him, and to ensure that the Horde does all that it was created to do. They are also tasked with protecting the Margog should that ever be required.