Earth is a world filled with Behemon's, demon-like monsters from another dimension. The main goal of Behemon's is to consume and destroy all life on Earth. Most Behemon's are very weak and harmless, but if left alone many will grow more powerful. However, there are many lethal and powerful Behemon's that could easily wipe out entire populations of people. Normal humans are unable to see Behemon's as they don't have the ability to. Humans that are able to see Behemon's are called Spirit Slayers. Spirit Slayers possess powers that are able to kill Behemon's and they make a living from killing Behemon's.