Fort Dawnguard, Dayspring Canyon, The Rift

The Fort dates back to at least the Second Era, when it was built by the Jarl of Riften. The Jarl's son, through his overly confident and adventurous pursuits, contracted vampirism. The Jarl refused to kill his own son, so instead commissioned the construction of the fort and contracted a large mercenary force, named the "Dawnguard," in order to keep his son quarantined.