Dayspring Canyon is a hidden valley in far eastern Skyrim, southeast of the Rift and accessed through an under-mountain thoroughfare found southeast of Stendarr's Beacon. It is the legendary locale of the storied castle Fort Dawnguard, and is now the destination of aspiring vampire hunters all over Tamriel. There, the Dawnguard, led by the famous vampire-hunting Redguard warrior Isran, has reclaimed that ancient sanctuary and intends to pick up where the Vigilants of Stendarr have, in their view, failed.
Fort Dawnguard can only be access by traversing Dayspring Canyon. Even fast travelling to the fort only leads to the canyon.