An Essence of Darkness is a Wisp that appears in the Lightless Oubliette in Coldharbour. Coming too close to one of these creatures will cause a loss of speed, and will temporarily disable the Light of Meridia.
During this quest, the Light of Meridia is used to provide illumination throughout the Lightless Oubliette. If the Vestige walks near an Essence of Darkness, they will extinguish the Vestige's light for about 5 seconds. It must then be activated to be used again.
For the most part, Essences of Darkness are harmless creatures. They will not attack the Vestige when approached, and doing so will instead only cause a loss of speed. When attacked, they will engage in combat, and have identical attacks to Wisps.