This door grants access to Shulva, Sanctum City. This changes in the Scholar of the First Sin edition of the game the Dragon Talon can be found on a body behind the 'Forgotten Door' unlocked with the Forgotten Key, at the bottom of the well in Majula. Upon loading, the Dragon Talon will be added to the player's inventory. Use the altar behind the bonfire to travel to the starting area.
This will involve traveling to three new areas (Shulva, Sanctum City, Dragon's Sanctum and Dragon's Rest) and defeating several new bosses. To access the content, defeat The Rotten in Black Gulch and proceed to the Primal Bonfire. In Crown of the Sunken King (again, the first DLC for Dark Souls 2, released on July 22, 2014), the player is tasked with reclaiming the crown that King Vendrick once owned.