There are three special doors in Dark Souls 2 called Forgotten Doors, which require a special key to open.. naturally called the Forgotten Key.
Be warned, these won’t necessarily be accessible early in the game, so make sure you’re able to defeat enemies in the listed areas.
Retrieving the Forgotten Key
Your first step is to get the Forgotten Key. Travel to Black Gulch. Go to the Hidden Chambers Bonfire, which is by a ledge near the entrance to Rotten’s Arena.
At the bonfire, go to the ledge overlooking a deep pit. Use a torch and follow the ledge to the right. You’ll find a lower platform that you can drop down to.
On this first platform is a Forgotten Door that you can’t unlock yet.
Drop down again to a second platform, where you’ll find a cave entrance. Defeat the two giants in this cave to retrieve the Forgotten Key. You’ll also find a chest in the back of this area.
Tip to help you beat the giants really easily, run straight to the back through the door where you find the chest. The giants can’t get in here, so range them until they die.
Now you have the Forgotten Key, which will open all of the Forgotten Doors, so where do you find all of the Forgotten Doors?
Finding and Unlocking Forgotten Doors
Black Gulch – On that first platform you dropped down to when getting the Forgotten Key is essentially the first Forgotten Door you can unlock. This door leads to one of the hiding spots for Darkdiver Grandahl, which you need to find in three locations in order to join the Pilgrims of Dark.
Majula – Once you progress to the tower in Majula, you’ll run into a man that will try to sell you a bridge for 2,000 gold. Purchase this bridge, and this man will show up again at the giant pit in the middle of Majula. Once you find him at the pit, he’ll then sell you segments of a ladder for various prices. When you buy his full ladder for 12,000 gold, you’ll be able to reach the bottom of the giant pit. At the bottom, you’ll find a short ladder going back up, where you’ll find another Forgotten Door containing three chests.
The Gutter – Starting at the first bonfire, go ahead through a door and drop down in the next room.Turn to the left and you’ll see two vases that kind of point to an area below you can jump to. Defeat the enemies out on this platform you jumped to, and look for a ladder leading up. At this point you’ll want to equip any anti-poison equipment you have. Run quickly through the poison spitters and unlock the Forgotten Door. There will be a lot more poison spitters though, so dodge roll a lot and break the ones in the center, and break the vase to retrieve Havel’s Armor, which was some of the best armor in the original Dark Souls game.