Wesen

Wesen (pronunciation: VAY-zin, Grimm: VES-sin; Germ. "a being", here "creature") is a term used to describe the creatures visible to the Grimms. They are the basis not only of the fairy tales that the Brothers Grimm have compiled, but also of the many legends and folklore from many cultures (i.e. Blutbad, Dämonfeuer and Wendigo).

Characteristics
Wesen are liminal, that is, they display two states of existence simultaneous within one physical body. Physiologically, Wesen are parahuman, exhibiting two distinct sets of DNA within the same system. When in human form, Wesen are, by all accounts, physiologically that way and thus can pass as human. However, certain aspects about them still sets them apart from the rest of humanity, such as the addictive effects of Jacine have upon them (which would otherwise be fatal for humans) and their susceptibility to the Yellow Plague. Every creature encountered by Nick Burkhardt are Wesen (excluding La Llorona and Volcanalis).

The community consists of both good and bad creatures ranging from Blutbaden to Bauerschwein to Mauvais Dentes. Individually, Wesen behave stereotypically in accordance to their own kind (i.e. the Blutbaden being fierce and vindictive, Siegbarste being dangerously vengeful, or Mellifers for their hive-like mentality and penchant for sending messages). Some Wesen follow ritualistic practices passed down through history (i.e. the Aseveracion and Roh-hatz). Other than the Grimms keeping the Wesen in check, humanity is only blissfully unaware of their existence.

Despite being para-human themselves, many Wesen eat humans (ex: Blutbaden, Wendigo, Coyotl, and Mauvais Dentes) or other Wesen (ex: Blutbaden eat Bauerschwein and Seelengut, Lausenschlange eat Mauzhertz).

Though it cannot be certain, it appears that ingesting Grimm blood can turn Wesen into regular humans. In Nick fights with Adalind and gets her to bite his lip causing him to bleed and some of the blood gets into into her mouth. A few seconds later, the Hexenbiest spirit inside of Adalind leaves her body causing her to become a normal person, and lose all of her powers. So far this is the only time that a Wesen became a human. Grimm blood may only affect Hexenbiests this way and not other Wesen types.

It may also be possible to turn Wesen into normal humans via drugs. Konstantin Brinkerhoff pioneered a series of medicines that would, supposedly remove the ability to Woge. While the experiments were undoubtedly a failure, it must be pointed out that Brinkerhoff insisted that the drugs would work if he'd been given the time to perfect the dosage and delivery system.