The Legions of the Nether are a mythical army with the capabilities of conquering any world they can reach.
The origins of the army are unknown; the creator, equally so. Some say when Herobrine came to the Nether, he got seven pigs and gave them some of his power. They became intelligent, at the cost of Herobrine's godhood. They went about the Nether, creating a massive army. Others say there is an ancient king in the Nether that raised an army long before Notch gave life to the Overworld. However, all stories of the legion are very vague. The Voice in the Darkness most likely had some part of it, as Herobrine created the Amulet of Shadows (Did he?) which possessed him soon after. It figured out Herobrine was evil enough, so left him on his own. Still, Herobrine was influenced by the Voice.