- The entry to The Depths is a door in Lower Undead Burg that you now can open with the Key to The Depths you acquired from the Capra Demon. The door is located near the base of the tower containing the lone Hollow archer.
- Going down the stairs, you will soon run into the first Hollow, but watch out, because there will also be another one coming from the opposite direction. Kill them both and move on to the stairs leading to a room full of tables and Hollows. Try to engage them one by one and be especially wary of those wielding a flaming stick.
- Go down the next set of stairs and kill the Undead Attack Dog. Up ahead is another dog and a Butcher; try to kill the dog first and do it fast, then use your superior speed to your advantage in your fight against the brute. After you killed them both, open the chest and get the Large Ember.
- Turn around and head down the stairs into the shallow water. If you keep to the right wall, you will not need to fight the two Undead Attack Dogs and just have to kill the single Hollow in the corner.
- Leave the water by walking over the debris and enter the room to your left. Free Laurentius of the Great Swamp from the barrel by rolling against it - do not use any kind of attack or you will not be able to trade with him, unless you pay for your Absolution.
- Leave the room and go straight ahead, but be prepared for there is another Butcher lying in ambush above the door, where you just freed Laurentius. Kill him and proceed to the next room, filled with shallow water. Watch out for the Slime on the ceiling, as it will drop down and hurt you immensely if you loot the corpse and do not attack it first. Be also prepared that a Hollow could attack you here.
- Now, turn around, backtrack to where you fought the first Butcher, and destroy the crates in the corner, right behind the Butcher's table. Jump down into the hole and find yourself on top of a ledge, overlooking a large room and the Giant Undead Rat mini-boss. Before you engage it, you should pick up the Spider Shield, found on the other side of the ledge. You can now attack the rat by either killing it via magic and ranged attacks respectively, or quickly through a lunge attack.
- After you killed the rat, loot every corpse in the room, one contains the Sewer Chamber Key to the only bonfire in the area. Take notice of the "water slide" at the one end of the room, turn around and leave through the pathway on the left. Take a left turn, and another, killing the rats on your way, and jump over the hard to miss pitfall trap. Take another left turn and kill the rats, or leave them be. Turn around and go up the stairs, but watch out, because a hostile rat will jump out of the crate up ahead and attack you, so literally keep your guard up.
- Continuing onward, you will run into a dead end with five rats and a corpse. Opposite to it is an open pathway, go through, and find yourself in a cage-like room, looking into the room where you just killed the Giant Undead Rat. Destroying the crates in the corner will reveal a hidden ladder which leads down to a door that you will soon be able to open from the other side as form of a shortcut.
- Go up the short flight of stairs and kill the Hollow. The corridor to your right will eventually lead back to where you encountered the first slime; the ceiling of this particular corridor is littered with them. Unlock the door to your left and rest at the bonfire. It is recommended that you revert to Human and kindle this bonfire, since a boss fight is soon to be had. If you do not have enough Humanity, farm the rats in the general area for more, it should not take very long.
- Go back to where you jumped over the pitfall trap and jump over it again. Ignore the right turn and go straight ahead, turning left at the next intersection. Try to lure the Large Undead Rats one by one into your corridor, while trying not to be hit by Soul Arrows from the Channeler. Also be careful of the rats, since these are bigger, stronger and receive a buff from the Channeler.
- Once you killed them all, take care of the Channeler, who would have else interfered with your upcoming boss fight (you can enter the Fog Gate you see here, since it only leads back to the sewers from whence you came). If you go straight ahead until you reach the far end wall, you will find another corpse to pillage, but be ready to be ambushed from the first two crates on the left.
- Now, return to the room where you killed the Giant Undead Rat and slide down, where you previously turned around. You will take some moderate falling damage, but the biggest threat as of yet lurks behind the corner to your left: your first Basilisk. With his breath, he can inflict Curse on you, and you really do not want to have that. Let him lunge at you and immediately back up around the corner. When he jumps at you again, kill him as fast as you can, so he does not get a chance to breath his curse cloud, potentially killing you instantly. (Or, when you see the drop, go to the far left, you will slide down a side path and get to the other side, and will never have to go down there. Then you can easily go to the dragon, or open up the shortcut.)
- For now, ignore the path to your left (once you drop down the slide in the middle, there are about ten more Basilisks in this area and some items. You should come back here, once you killed the boss.) and go ahead. Be careful, since there are three Giant Undead Rats waiting for you around the corner. Also, if you are in Human form, the Black Phantom Kirk Knight of Thorns will invade your world at this point, so be prepared. Go up the flight of stairs you come across, go straight ahead to the other side of the room, and up the pathway you will find. Opening the door will open up the potential way back as well as pose as a shortcut to Blighttown.
- Use your chance to rest at the bonfire, and restock your flask charges. Go back down from where you just came and head towards the door at the end of the room. You cannot open it just now, but you can talk to vendor Domhnall of Zena, who sells the Bottomless Box and Gold Pine Resin, the latter of which is quite helpful against the Gaping Dragon, but only if you are not using the Drake Sword or Astora's Straight Sword, since you cannot apply it to those two weapons.
- If you are not interested in any of his goods, make a sharp turn to the right, just before you get to him, and go through the doorway. Turn right and right again, and go down the stairs you come across. At the next set of stairs, just between the stairs and the pillar, you will find Solaire's Summon Sign if you are Human and have talked to him at the bridge in Undead Burg. On the other side of that pillar, you will find Lautrec's summon sign if you freed him, before killing the Bell Gargoyles. You can have summoned both at the same time for this fight (Remember: If you die during the boss fight, you first have to revert to Human form again, or you will not be able to summon them). Up ahead you will find the Heavy Crossbow. Going down the last set of stairs will pit you against the Gaping Dragon. When you kill him, you will acquire the Blighttown Key, which will open the big door next to Domhnall.