The Devourer

Known by different names in different regions the Devourer is a creature pure energy. It's form appears to be pitch black and mabye of a gooey substance. These creatures were the cause of the barrier that was erected hundreds of years ago. Since then most have died of starvation or from hunting. Devourers are called this because of how they hunt, survive, and reporduce. When a Devourer encounters a creature in the wild they attack and infect it. Creatures affected by a Devourer gain a huge surge in energy; even a creature on the brink of death is suddenly capable of down multiple foes. This power, however, comes at a cost. The energy gifted to the infected continues to rise higher and higher without end. This keeps going on until the creature's physical form can't handle the amount of energy and they begin to tear apart at the seems. If nothing is done than the infected keep tearing apart until they are nothing more than a pile of black goo, now transformed themselves into a Devourer. 
The only way to prevent the transformation after being infected is to die, but even then that may not always work. Because of how the creatures exist they infect not only the beings body, but also their soul. Because of this if a creature isn't killed within the first couple seconds of getting infected, than they are doomed to irreversibly become a Devourer. No creature, from the lowest Beast to the greatest Dragon is immune to this effect. Once transformed, the new Devourer loses all sense of self and devolves into a mindless beast. Devourers survive by constantly infecting other creatures as a means to alleviate their own building energy. If they go too long without infecting something their energy builds higher and higher until they reach a critical point. At this point the last of their body is consumed by power and they cease to move. 
From this point on they are essentially a black puddle on the ground. As long as no creature touches it than they are unable to move or take any action. Anybody hunting a Devourer can spot these puddles and easily contain them within something like a waterskin or pouch. From there they can burn the remains or dispose of them some other way.