Go beyond the story in this RWBY supplementary series, delving into the various forces that shape... the World of Remnant

The narrator states Grimm have been threatening humanity for as long as humanity has known with no knowledge of their origin. Grimm are noted to only target humans, with other animals only fighting them for territory, and to not require sustenance. Negative emotions are known to attract Grimm, making defense difficult as panic attracts more of the creatures. It is stated that research of the Grimm is difficult because it is hard to keep them in captivity and when they die their bodies slowly disintegrate. Finally, the Grimm are found to be capable of learning and becoming more effective killers with age.