The Pit | Dark Souls 2 Wiki

Journeying In The Pit

Surviving the Fall

• You need at least 1346 HP or the Silvercat Ring, which is purchasable from Sweet Shalquoir for about 13,400 souls. While this may sound like a lot, especially early game, it is necessary to survive the initial drop.
• Stock up on Estus Flasks. The drops, especially the first, will still take a chunk of health. Make sure to refill your health before every drop.
• Fall Damage Reduction Equipment: Lion Warrior Cuffs, Flying Feline Boots, Jester's Tights. All these items and the ring can be worn together to greatly reduce fall damage. If you unequip all weapons and any armor that does not reduce fall damage, it will also lessen the amount of fall damage taken. 
• To reach the Forgotten Door in The Pit which has a visible ladder leading to it, the player must drop down in a particular path that involves a harsher drop than otherwise.  Using the platforms to strategically fall shorter distances will put the player onto platforms that leave no path to safely jump to the the ladder that leads to the door.
• Do not jump on the thin narrow fastened beams that look to be of questionable physical integrity to support the impact of a falling human-sized body.

Ladder Down

Alternatively, another way to descend the pit is to purchase a ladder from Laddersmith Gilligan. He offers three ladders, each more expensive than the last. 

• The cheapest ladder will only go half way down to the first plank, but it could be all you need if you have high health and/or the Silvercat Ring.
• The second most expensive ladder will take you down to near the Grave of Saints entrance. From there, you can drop down a few more levels to a platform just outside the bonfire at the start of the Grave of Saints. Be advised that you will still need a lot of HP and/or the Silvercat Ring (or other fall damage reduction items) to reach the entrance. A separate "drop path" will put you on a different platform at the bottom of a ladder.
• The most expensive ladder gives you easy access to the Forgotten Door, far down in the pit. The key to the door is found in a hidden cave in the Black Gulch; see the next section for instructions on how to attain it.
• Continuing to drop down from the bottom of the ladder will put you on yet another ledge which meets up with the Wooden Bridge section of  the Grave of Saints, which eventually leads to The Gutter.

Opening the Forgotten Door

The key to The Pit's Forgotten Door must be obtained in Black Gulch. Proceed out from the Black Gulch Mouth bonfire past the poison statues, several black oil pools, and giant cave worm/bugs that emerge from the holes in the wall.  Drop down the ledge (use a torch for visibility) in front of the second giant worm and you'll find a locked door.  To gain the key, drop further down the ledges and go down the tunnel to find two giants (there will be at least two; if you've summoned an NPC or another player, there will be more).

The giants will drop the Forgotten Key which opens the Forgotten Door in the pit, and also opens the locked door on the ledge on the route to where the Forgotten Key was found in Black Gulch.  Behind The Forgotten Door are there are 3 chests containing Great Lightning Spear, Witchtree Branch, Witchtree Bellvine, and Soul Vortex, and a body holding the Dragon Talon.