Dawnguard revolves around an ancient prophecy predicted by an Elder Scroll. The Dawnguard, an organisation dedicated to hunting vampires, are reformed to attempt to stop the return of a powerful clan of Vampires named the Volkihar... The Volkihar's leader, Lord Harkon, intends to use an Elder Scroll to permanently blot out the sun. The player can choose to either aid the Dawnguard or join the Volkihar, becoming a vampire lord themselves in the process. (wikipedia.org)