Geier

A Geier (plural: Geier; pronunciation: GY-ər; Germ. "vulture") is a highly sadistic vulture-like Wesen.

Characteristics
They have an innate ability to move through trees, and are very adept at ambushing their prey in forest terrain. These creatures are harvesters who take pleasure in the pain of others. Geiers gut humans for their organs and blood, to use as homeopathic medicines and aphrodisiacs within the Grimm creature community, and are responsible for procuring such agents for the benefit of Wesen kind. This gruesome practice may be referenced to an actual vulture's role as a scavenger, feeding on the carcasses of people and animals who died in desert areas.

Biology
When a Geier transforms, the creature's hand elongates and grows sharp talons, which they use for defense or attack. Their feet grow an additional talon on their heel making attacks from the air more effective. They also seem to develop a hook-like nose, resembling a vulture's beak. They attack from above patiently awaiting their opportunity. They appear to be no stronger than ordinary humans, but are certainly much more agile, and are able to fight a Grimm on near equal footing (but only when ambushing, in a fair fight they are vastly inferior).

Geier are a communal race, living and working with groups of their own kind.

A Geier is seen briefly in, watching the fight from above in the rafters.

Grimm Diaries
The book entry says "Geiers are the vilest of all; they harvest human organs while their victims are still alive, seeming to take pleasure in the savage pain they cause. Carnage on the front lines (of battle) has provided them with a seemingly endless supply of viscera for their terrible trade."