Praise the sun. Our Dark Souls Walkthrough provides tips for surviving the adventure, a guide to the entire world, and hints for slaying bosses.
When you arrive in Depths, you'll descend some stairs and then arrive at a large balcony area. There are some obvious enemies ahead of you, as well as one that may try to ambush you from behind. Block their attacks and then easily counter with some blows of your own. Once the ledge is cleared, check back behind the stairs you just descended to find a Large Soul of a Nameless Soldier.
Once you've gathered that loot, it's time to head to the far end of the ledge. There, some stairs descend back toward the right and toward some waiting enemies brandishing torches. Take them out as you see fit. You can retreat up the stairs to force them to come at you single-file, if you like. Be careful, though, because they'll come at you from basically all sides of the staircase and it's easy to forget to look behind you.
After you clear the area of those enemies, you can descend toward the left where it looks like a blacksmith is pounding on a forge. That's actually an enemy pounding a slab of meat with a cleaver, not a friendly blacksmith. As you approach, some of the poisonous hounds will rush to meet you. Block their attacks and pet them with your favorite melee weapon. Then you have to deal with the ogre. He swings a huge cleaver. It does a lot of damage if it connects, but the ogre is slow and you should be able to easily dance out of the way before moving in and hitting him with your sword a few times. Then you should back away again and repeat as needed.
When you defeat the ogre, check the treasure chest against the wall, next to the barrels. You'll find a Large Ember to reward you for your troubles. Breaking the barrels also reveals a hidden opening. You can drop through that opening in a moment, but for now you have other things to do. Since you have the ember, you should head back the way you came, keeping to the lower level.
As you proceed through the fetid water, watch for more of the hounds and more of the torch-carrying goons. It can be difficult to see, since the wooden planking overhead gets in the way of the camera, so make sure to lock onto a target and circle around with your shield raised so that you don't lose track of a threat.
When you near the edge of the area, another of the goblins with a set of cleavers will surprise you. This one drops from the rafters to attack. If you haven't cleared out the other enemies, this can make for a hectic fight. Even still, the goblin has a way of bringing a nasty surprise to the occasion. Move carefully around him and make sure that you avoid his cleaver. Dance out of his way until he slams the ground in front of him with the cleaver, which leaves him briefly stunned and lets you attack safely. Don't lose your cool and you should come through the scrape in relatively good shape.
After clearing out the enemies, head toward the back wall and up some steps. You'll come to a split in the path. If you head left, you'll find a store room full of barrels. A man in the back corner is asking for your help. You can roll through the barrels (there's no need to use a weapon) and free him. Talk to him and he'll reveal that his name is Laurentius, of the Great Swamp. He now is in your debt.
Return to the branch in the path and this time follow the other route. You have a few points of danger along this route, but it's an important one to follow if you want to reach the bonfire. As you head along it, you won't have anything to worry about first. Proceed along a stone corridor until you descend some stairs to a small, square room. To the left, there's a corpse resting on a ledge. Resting on the corpse is a Soul of a Nameless Soldier, but it's a trap. If you approach, slime will drop from the ceiling and try to absorb you. An enemy will also attack, so it's easy to take a lot of damage here if you fall for the trap.
Head along the passage until it takes a right turn that fines you looking down another long passageway with a skeleton near the end. This is a treacherous area because there are more of the slime blobs on the ceiling. They'll drop to hit you if you move slowly, so you need to either rush past them in a dash or you need to lure them down. A dash is risky but can definitely work if you head along the middle of the path to avoid the first two slimes and then dodge left or right to avoid the third one. Then you can roll forward take out the enemy at the far end of the passage before turning around and pelting the enemies with firebombs or magic. Fire works really well on the moving slime and it will occasionally drop the Large Titanite Shard that you need for upgrades.
The door at the end of the passage is locked (and leads eventually to a bonfire), but you can turn left from there and descend more steps to reach a grated area. Here, there are some crates you can destroy to reveal a ladder, but if you descend the ladder you'll only find a door that must be opened from the other side later in the stage. For now, ignore this lower route and remain along the higher rout to reach a tunnel that leads to a dangerous 'T' in the path. Here, you can either head right toward a swarm of waiting rats or left and take your chances with more of the poisonous gelatin. Neither route is likely to end well for you unless you prepare yourself and fill your health. You can help yourself with the rats by luring them toward you with arrows, though. It's best not to rush them all at once if you have a choice.
Once the rats are cleared out, check the nearby corpse for a Greataxe. Then turn and head back toward the left. You can actually run past the oozing slime, since it's on the floor and only moves slowly. Past the slime, though, you'll find crates. A rat bursts out of one of them, so hack it apart quickly and round the bend. There, another rat appears and runs from you. Watch where it goes, down some steps to a fetid pool of water. You can fire arrows at the rats from your vantage point to force them back up the steps, enabling you to destroy them one at a time while they pose a smaller threat.
After clearing the lower area of rats, descend the steps and look to the left. There's some grating positioned there, with a corpse lying on the other side. Make sure you kill any rats that might have survived your arrow attacks, then check the corpse to obtain the Sewer Gate Key.
Backtrack the short distance to the locked door. The key that you just found will work on that door. Use it to open the door, then head down the waiting corridor. At the end of said corridor, you'll find a bonfire waiting for you to use it. This is terrific because you're in an area where it's easy to farm the Large Titanite Shards (by killing the slime that appears on the ceiling in the corridor near where you find the bonfire) that you can use to continue upgrading armor. Depths is good for something after all!
Now that you've lit the bonfire, it's time to resume your exploration. While it's easy to feel good about your progress, though, you're coming up on an area of the game that can really try your patience. It's important to be extremely careful as you continue exploring.
Start by backtracking to the area where you fought the first goblin with the meat cleaver. If you've killed him once, he's no longer there. You'll still have to deal with the dogs and other enemies, however. When you reach the area, break the barrels to the right of the treasure chest that you emptied, revealing the secret passage again. Now drop through the passage to land on a high ledge along one edge of the room.
Ahead of you, the ledge drops to lower ground. Don't drop down just yet, though. Instead, look around. Ahead of you, there will be a giant rat roaming around on the lower level. You can fire arrows at him from your vantage point on the high ledge and inflict damage. It's possible to kill the rat without ever needing to drop down to his level. Then you should head around, still on the upper level, and check a glowing point on a corpse to obtain the Spider Shield. After you have that item, go ahead and drop down to where you killed the giant rat.
On that lower level, collect the Humanity that the giant rat will have dropped. You can also find a Soul of a Nameless Soldier near one corner of the chamber, not far from the top of a waterfall. Keep that waterfall in mind as you will be returning later, but for now you should look along the opposite side of the room and there's a narrow passageway. Start down it and grab the Soul of a Nameless Soldier from a corpse. From there, you have a choice. You can either head left, where a rat wants you to follow it to your possible doom, or you can continue straight ahead and fall down a shaft.
You do not want to fall down the shaft, especially not just yet, so begin by heading along the left route. The rat will lead you to a 'T' in the path where a second rat waits. At that intersection, you can choose to head either left or right. Start by going to the right to find another intersection. To the left, there are some rats waiting along with a magic user. If you're ready for a group of rats, you can head in that direction. Or you can lure the rats toward you and eliminate them, then start forward again.
There are a lot of rats along that route, around four or so in the nearest area. There's also a spellcaster who hangs back and will try to pelt you with magic. As you follow him, note that more rats will break out of the crates to attack. Be ready for that and deal with them while you avoid the much more lethal magic from the magic user. Then tend to him (he won't come back after you take him out now, thankfully) and investigate the area beyond him for the Large Titanite Shard on a corpse in the corner of the high ledge where you're now positioned.
Note that along the left side of this ledge, you can also find a glowing, mist-shrouded doorway. It doesn't lead anywhere that you haven't already been at this point, but go ahead and pass through it so that it's not a distraction later. Then head back the way you just came and return to the area where you fought the giant rat. Along the edge of that chamber, there's a steep slope where water rushes down into a gap in the floor. To the left of that is a stone surface that also descends in that direction. You can slide down that stone slope and if you perform it properly and veer left, you should drop safely to a lower area.
This area is the same general area where you would have appeared if you dropped through an opening that we suggested you avoid earlier, but you've entered it by a safer route. Now you can find out what to expect from the area.
Begin by heading around carefully and watching for what looks like a large frog. The frogs seem like no big deal, but be careful because they can inflict you with Curse. If you are cursed, bad things happen. Your health is halved and that remains the case even if you die and return to a bonfire. You'll have to use an expensive Purging Stone to cure yourself, if you even brought one (you should have a few in your possession if you've been following this walkthrough from the start, but it's still no fun to have to use them). To fight the frog, either hit it with fire magic from a distance or let it get close. Then dodge back out of the way, wait for it to breathe noxious fumes, then hit it as it starts forward again. Avoid standing in the vapors or you will likely be cursed. Your likelihood of being cursed increases if you are not in human form, as well.
Head forward along the main corridor after you take care of the frog (don't head along the left corridor or you'll find a bunch more cursed frogs). Watch out for a number of rats that are in the area along the main route, as well as slimy blogs that drop from the ceiling to try and surprise you. Remember that you can take them out with fire attacks. They'll go down quickly to that particular element.
Continue around the passage and you'll find grating through which you can pass. There's a hole in it. On the other side of that grating, you'll find a merchant who you can talk with to purchase items such as the Bottomless Box (for inventory management at bonfires) and the Crystal Straight Sword and Crystal Shield. He also offers the Crystal Greatsword and the Armor of the Glorious. The armor is the most expensive item, with a cost of 15,000 souls. It's clear that this merchant is after some real riches.
Purchase what you like, then turn around and pass back through the grating. To the left just after the grating, there are some stairs you can climb to an area outside the castle that you're exploring at present. If you head up those stairs (which you shouldn't do just yet), you'll find the route that leads to the area boss. Before you do that, though, it's time for a little bit more backtracking.
Head back to the lower chamber where you fought the frog. Continue left and you'll find another set of stairs that leads upstairs. Follow those steps to find a doorway. You can open the door to create an important shortcut. Head through the door, climb a ladder and you'll appear in an area that should be familiar to you. It's the grating area that overlooked the large rat that you killed previously.
Now you can simply turn around, head back down the hall, kill the waiting skeleton, take a sharp right and head down to the room with the grating, descend the ladder and then head down the steps through the door that you recently opened. If you killed the frog already, it'll be gone. Remember to watch for the rats and slime, though. Take the next opening to the right and climb the stairs to reach the outside area again.
You're now ready to proceed to the right, through that area. Fortunately, there are no waiting enemies. Just head around the castle tower until you find some stairs that descend one level. Head down those stairs and then to the right. There are more stairs there. At the top of the staircase, you'll see a patch on the ground where you can summon Soltaire if you like. You can head left around the corner from there to find a Heavy Crossbow and a Heavy Bolt waiting on a corpse, if needed. Then you can return and descend the stairs to reach a mist-shrouded doorway. If you pass through that doorway, you'll find a boss battle.
This battle should go fairly easily for you. The monster that you'll fight, though huge, moves slowly and doesn't have any projectile attacks. You can avoid taking damage almost entirely just by staying at a distance, but the best way to kill the dragon (naturally) is to get in close and hack at it a few times, then retreat to avoid taking damage.
When the dragon first appears, it doesn't look like much of a threat at all. Then you see its full size emerge from the water and it becomes much more imposing. Your first order of business should be to hack of its tail, which should only take a few hits if you have a good weapon equipped. The first time you do that, the dragon will drop the Dragon King Greataxe. Be sure to pick it up and then return your attention to the battle at hand.
The dragon likes to lift its torso back, then slam down on the ground and spread out a yellow mist that you definitely want to avoid. When it finishes doing that, you have a brief moment and then the dragon will charge you. Definitely get out of the way. Try to stay on dry ground rather than wading through water, since you'll be able to move better. Note that sometimes the dragon will lift up from the ground and fly a short distance. When you see that starting to happen, roll like crazy to get out of the way. If the dragon lands on you, the damage it will inflict is significant. If it manages to grab you with a swipe of its paw (which shouldn't happen unless you get especially careless), it can swallow you and inflict severe damage, but that attack is easy to avoid as long as you don't get greedy about hitting the dragon too many times in a row.
For the most part, it's a pretty easy fight that ranks among the easiest in the game. Just move in and hit the dragon in the flank two or three times with a quick weapon such as the Drake Sword, then back quickly away as the dragon retaliates (it can bend over backward to hit you if you just remain in one place and keep hacking). It's an easy process to perform and to repeat throughout the fight until you win.
Once you defeat the dragon, you'll receive the Blighttown Key. It's also safe to check the corpse that you've likely noticed in the area, from which you can pillage the useless Standard Helm, Hard Leather Armor, Hard Leather Gauntlets and Hard Leather Boots gear (no wonder the poor sap died!). Then you can head back through the dungeon to the area where you found the merchant. Nearby, there's a locked gate that the key will open. It leads to a new area: Blighttown.