The game follows the story of Eric Bane, a newly turned vampire suffering from amnesia. He learns that his transformation is not complete and that, if he does not drink the blood of his sire, he will mutate into a mindless ghoul. In order to avoid being such a foul creature, he receives missions to drink ancient vampires' blood. However, an angel in Eric's view appears to guide him to the righteous path, and ease intolerable pain that comes from not drinking appropriate blood as it disappears. With a question of this vision, he tries to recover his deleted memories and his true-self 'vampire'.
Source: Wikipedia, "Dark (video game)", available under the CC-BY-SA License.