There are three ways to reach the Forgotten Sepulcher. The first is from Stilt Village using the Teleportation Rune. The second way is accessible via Slumbering Sanctuary and the last way to access it would be via the Graveyard.
I personally found Stilt Village the easiest way to progress to this area.
I've linked a video below as well as a full playlist showing you how to get all areas/runes.