The Green Legion refers to the contingency of forces that were led by Genther Stormloft during the Great Purge. Of all the forces tasked with eradicated magic users, Stormloft was the most dedicated to the cause; because of his zeal, he and many of his forces followed the mage Atury through the Dreadgate and ended up in the land of Errod.
Dreadgate Lore
This faction corresponds directly to the Champion Craft in Dreadgate (TCG), also known as the green deck.
Canonically, the units in Champion are called the Green Legion. This is because many of them are of Gruenish descent, who wore green uniforms and armor.