The Devouring Earth are simple natural objects and creatures that were given sentience by a powerful god-like being known as the Hamidon to carry out its desire of destroying the World of Man.
The Hamidon discovered that the more simple the creation, the more powerful it could be. Thus the most powerful creations are animated rocks, crystals, and simple plant-life. However once it turned to more complex life such as insects, the Hamidon lost much of the individual control and could only instead guide the collective hive mentality.
The Devouring Earth have no concept of right or wrong. It sees all other sentient life as a threat to its "god" and seeks to destory it completely.
(Please see their Paragon Wiki entry for more information.)