The Dark Fae are one of the two Fae clans (the Light Fae is the other) in the show and are made up of members who can be at best described as free-spirited, and at worst pure evil. In this they might be seen as equating with the chaotic spirits of many mythologies, as opposed to those of a more ordered tendency. The Morrígan is the Dark Fae leader of the local territory. Vex, a Mesmer, serves as her hitman, carrying out punishments and executions.
Many of the Dark Fae are portrayed as amoral and corrupt, operating in activities that others would consider criminal and cruel. While many members of the Light practice restraint and control, many Dark revel in their abilities; embracing what their powers achieve for whatever means they choose.
While they do dispose of their human kills, the Dark are known for leaving a higher body count than the Light, and according to The Blackthorn have no qualms with murdering for pleasure as well.
Essentially what sets the Dark Fae apart from the Light Fae is their willingness to use their unique gifts purely for personal gain regardless of the consequences to those around them, especially humans.
Like the Light Fae, the Dark Fae have their own group of leaders and elders. While the The Dark title their leaders with the names of fallen warriors; while the Light name their leaders after sacred trees.
The Dark seem to hold the monopoly on the industrial and criminal elements of the local territory. Everyone is expected to pay a quota to The Morrígan from their businesses, and punishments are considered "unpleasant" if the quotas are not met.
The Dark appear to follow no rules but their own; unlike the Light, who follow a strict set of rules. They are, however, respectful of the Old Laws, such as the Blood Laws, and those governing a territory. Despite animosity with the Light, it is made clear several times that the Dark collectively have no interest in another Great War.