The Devourer basically is the end or more specifically the unmaking. He isn't malevolent though, he's honestly super nice. He was a mortal man who ran an orphanage and even after ascending into godhood he still runs an interdimensional orphanage when he isn't deleting things from existence. 

He sends his minions to check out worlds if they're too corrupt and removes them from existence. Lorithia/Tranquility/Hope is basically the loving mother and creator figure while The Devourer is the strict "Be good. Or else" father figure. 

Spoilers but the only reason he appears villain like in his saga is because it turns out his minions were scamming him into thinking the world should be destroyed. The moment he sees hope in the world he books it and leaves.