From ROI Wiki

The Horde is an army of cybernetic creatures created by and ruled over by the Margog. It consists of four basic types of creatures: Garac, Imp, Vergon and Geffen. These creatures are organized into legions of ten thousand each and fight their battles in a rage berzerker style, overwhelming the enemy with sheer numbers. The estimated total number of Horde believed to exist tops 8 million. The Horde, while a seemingly unstoppable force, also has an Alkalies Heel. Fearing their strength, and that they might turn on him some day, the Margog installed a kill script within their design that, should he ever die, they will all die as well. As such, the Horde are willing to do whatever it takes to ensure that he lives and survives, no matter what, causing them to form a nearly impenetrable ring of protection around him at all times. However, despite their best efforts, this ring of protection will fail, and the Margog destroyed, ultimately causing the entirety of the Horde to destroy itself in one fateful, fiery, terrifying conflagration.