To reach the Untended Graves secret area, you must defeat "Oceiros, the Consumed King" Boss. This is relatively late in the game. Oceiros is Boss thirteen of eighteen. There is also a bonfire in the Boss room which you can use to fast travel. Open the door where the Oceiros Boss fight took place and follow the path into the next room. There is a secret wall behind a chest (end of the room). Hit the wall to make it disappear. You can then drop down into the Untended Graves area and will get the "Untended Graves" achievement.