THE SPIDER FLY PATH
Fall through the floor. Turn right, kill the spider fly and fall down the hole behind it. Here, you'll need to dispatch half a dozen more of the creatures. Pillage four corpses for Lifegem x 5, Old Growth Balm x 1, Dung Pie x 10 and Poison Throwing Knife x 20. Feel free to destroy all of the egg sacs, but none of them contain anything. And beware of equipment breakage. Climb the ladder at the top of the stairs.
Open the chest in the room at the top of the ladder to find Flynn's Ring x 1, which lowers equip load and increases physical attack. Exit by hopping down the only exit, and you'll find yourself back in the hallway that lead you there. Go back down the hole and take a right this time.
THE SPIKY SANCTUM KNIGHT PATH
Stand at the top of the stairs and use a projectile weapon to pick off the sorceress at the far end of the room tossing hexes your way. The rooms ahead are filled with spikes on the floor. Hug the left wall and walk down to where it takes a sharp left. There's a ghostly Sanctum Knight waiting there for you, but he can't see you unless you go too far. Equip a projectile weapon, and look to the ceiling opposite the left wall. Hit the Stone Switch there to retract all spikes.
This next part requires speed and a bit of luck, but it pays enormous dividends. In their ghostly form, the Sanctum Knights are near impervious to damage, and there are six of them ahead of you. But there's also a room full of armor that can give them bodies. Here's how.
Run down the stairs and through the first doorway on your right. Avoiding the knight in the room, keep the wall to your left and climb the stone ladder in the far left corner of the room. Turn left, run across a bridge and through the doorway ahead of you avoiding another sorceress and ghost knight, and cross the bridge. Make your way across another bridge and down the staircase to a room with three more ghost knights. Do not engage them. Run around the periphery of the room smashing sets of armor as you go. The easiest way is to roll into them. Now, every ghost has a body — and during our playthrough, the three knights in the room were dead and gone by the time we were done smashing sets of armor (presumably because they fell down the room's bottomless pits). Before you leave the room, make sure to collect the Eternal Sanctum Key from the corpse in the far right corner of the room.
Walk back up the stairs, enter the room you ran through and kill the knight and the sorceress. She deals dark damage, so you can equip an item like the Transgressor's Leather Shield, which has 90 Dark resistance, to take her on. Thanks to your tireless efforts, the entire area is safe to explore. Open the chest in the room and immediately raise your shield, because it's boobytraped. Collect Lloyd's Talisman x 5.
Walk back to the doorway that leads to the stairs, turn left and jump over the broken staircase toward a chest with Bloodbite Ring x 1, which increases bleeding resistance. Walk off the side and pillage the corpse for Torch x 3.
Ascend the ladder again. About halfway across the closest bridge, look toward the round rotating door on your left. Shoot a Stone Switch through a hole using a projectile weapon. Hop off the bridge and shoot it again through the little hole from the ground. Climb back up the ladder, walk to the far side of the bridge and shoot the switch one more time. Hop back down, enter the room and open the chest for Denial x 1, a miracle that allows you to "challenge fate one time when HP reaches 0."
Walk to and hit the Stone Switch in middle doorway that leads into this room. That'll open a stone door just beyond the ladder with a chest containing a Dragon Stone x 1, which is "used to operate the Eternal Sanctum contraption." Pillage the three corpses for Twinkling Titanite x 3, Holy Water Urn x 5 and a Human Effigy x 1.
At this point, you've got a choice to make. Having collected the Eternal Sanctum Key, you can make your way to an optional area called The Cave of the Dead. Or you can continue down the critical path to confront the DLC's ultimate enemy. If you're up for the adventure, head to Cave of the Dead.
Walk through the first room in this area and descend the staircase at the far end. A knight will be waiting for you, but he's got a body, so killing him will be relatively easy.
Stand at the base of the steps facing them and fire a projectile at the Stone Switch to its left. Walk back up the steps, return to the trusty stone ladder. At the landing at its top, where once your path was blocked ahead, now it's open. Jump across the gap through the door you just opened. Continue around the corners to pillage a corpse and collect the Sanctum Crossbow x 1. Light the bonfire here in Hidden Sanctum Chamber .
Descend the steps at the base of which you activated the last switch, fight the guard there and proceed through hallways until you come to a room with two staircases. Pillage the corpse for Old Growth Balm x 1.
Down the next set of stairs, you'll be invaded by Jester Thomas. He's got an insane amount of powerful pyromancies, so equip items (like a Flame Quartz Ring), armor and shields (the Grym Greatshield offers 100 percent fire resistance) to protect you. He's also adept at rolling to avoid projectile damage, but you can do the same to him if you keep your distance when not attacking up close.
Keep your guard up, roll to avoid his fire attacks and time your weapon hits strike to right after he concocts a pyromancy. That's when he's most vulnerable. He'll also invoke a healing miracle, which you can use to gain health, too.