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 Blighttown after using the key that you obtained by defeating the Gaping Dragon, the first thing you should do is equip the Spider Shield that you obtained in the Depths area. It provides powerful protection from poison attacks, which are common in the coming dungeon.
From the entrance, head left along the stone rim and you'll reach the top of a red ladder. Descend it. From the base of the ladder, start forward and look for the top of a wooden ladder to your left. Descend along that to a lower platform and then start forward to where an enemy will be waiting. It is a powerful enemy, but slow.
For this particular enemy, your best strategy is to let it attack. It will typically use a heavy vertical strike and follow it up with a swing or two. Block or dodge the initial strikes, then get around behind your adversary and stab him in the back a time or two, or just hit him quickly with your weapon a few times. Then repeat the dance. It's possible also to easily lure him to the side of the wooden planking and then let himself rush to his doom by way of a fatal fall. Just make sure that you don't make the fall yourself. Note that sometimes the monster will stop to bellow and you can get in a few easy hits, but you should be ready to back away when he finishes his bellow or he'll possibly hit you with a melee attack. He can also break your stamina down pretty easily if you take too many hits, so try not to let the monster hit you too many times in close succession. He often drops the Dung Pie when defeated, and less often the Large Club. Sadly, he will reappear in the area if you use a bonfire.
Continuing down the planking, you'll soon encounter a second of those same monsters. Don't push too far forward as you fight him, though; enemies below may notice your fight. You might not see them, but they can run around, find a ladder and climb up to give you trouble. You really only want to fight one enemy at a time, where possible. There's a third of the huge monsters after you defeat the second, as well.
After you've taken out the third of the goblins, you'll meet a new type of enemy. This one is faster and lizard-like. You have to treat it with more caution, but it's not a real threat if you're careful to block and counter. Keep your distance and let it swing its weapon at you, then move in for a killing series of blows to make the job easy.
Past the first of those more agile enemies, you'll soon find a more dangerous threat. There are some enemies nearby that are obvious, and there's one that will throw poison-tipped daggers. If these hit you, Toxic effect will take place. Your life meter quickly drains unless you use the Blooming Purple Moss Clump, found in Darkroot Garden when you defeat the walking tree roots. You can also buy it from the merchant near Firelink Shrine (up the tunnel that you explore on your way to Undead Burg). Make taking out any dagger-tossing enemies your priority so that you have to worry less about the toxicity.
When that's tended to, resume your trek forward. You can head right here, or left. If you head left, some enemies will rush you. Defeat them, then look down over the edge of the ledge that you have reached. You'll see a lower platform with a skeleton lying on it. The only way to reach that platform is to make a running jump from the corner of the higher ledge to the beam that extends from the lower ledge. Then you can walk along it and investigate the corpse to obtain the Iaito blade. Note that it's a tricky leap to make and you'll likely die if you fail, so keep that in mind and decide just how badly you want the sword.
Drop down from the right side of the platform next and you can head a few steps in that direction to find something wonderful: a bonfire. Naturally, this is a terrific base of operations as you consider your journey through the remainder of the area.
There are two direction you can follow from the fire. One is to head away from the fire, with the ledge where you found the Iaito blade to your back. That route actually leads to some ladders (positioned just to the right along some wooden planking) that you can climb to reach the overhead series of planks where you first started the level. It's the route you would have taken down to the bonfire if not leaping to the platform for the weapon. Along that route, keeping to the high road, you can find the Large Soul of an Unknown Soldier. The way to proceed through Blighttown can also be found if you head in this direction from the bonfire and just keep to the low road along the stone bridge (where some dogs are standing guard), but before you continue your descent you might want to explore in the area to the other side of the bonfire.
Your other option from the bonfire is to head back in the general direction of the high ledge where you found the Iaito blade. Head around the base of that ledge and you'll find the top of a ladder. You can peer over the edge to possibly attract the attention of some enemies lingering below. Let them climb up to you and then hack them apart, or drop down with your weapon ready to manage a damaging surprise attack (though you'll take some damage yourself because of the height involved). Descend the ladder and walk along the ledge they were guarding. On a beam that extends from it, you can investigate a corprse to obtain three of the Blooming Purple Moss Clump. Then you can return to the base of the ladder.
Now from the base of that same ladder, head up a small ramp and to the right to find yourself at the start of some rickety footing. Before you start across that, you should know that it will sway gradually when you put weight on it, swinging left or right like a wooden arm. Don't let that freak you out as you head slowly across the shaky footing, and don't let an agile red hound startle you as you reach the halfway point.
A word about the hounds here: they're smaller and faster than the ones you may have already encountered elsewhere in the game. Red in color, they can sprint fairly quickly and as they get near they may also try to roast you with fire. You're better off not facing them directly but (when the level architecture permits it) approaching them from the side instead. You may also try picking them off with projectile weapons.
When you reach the far side of that rickety ledge, you can drop off the left side and double back a short distance to grab Humanity from the corpse that you might have noticed lying off to the side. Then you're done in this direction and you should return to the bonfire.
Back at the bonfire, head in the direction where the ladders lead up to the area entrance from the Depths. If you haven't found it in this direction, note that you can climb a ladder and work your way past where an enemy is trapped in a stone vase. Kill the enemies there, then look to the right. You can drop from your current position to a lower ledge in that direction, then climb up another nearby ladder (after defeating more enemies) to find some excellent gear with terrific resistance to poison-based attacks: the Shadow Mask, the Shadow Garb, the Shadow Gauntlets and the Shadow Leggings. Drop down from that ledge, kill an enemy and check around the wooden walls to find two Soul of a Proud Knight items resting on corpses spread around the same general vicinity.
Once you have that gear and those items, you're going to want to stick to the lower route that leads across more of the stone bridge (no ladder climbing necessary). In that direction, you'll find more of the fiery hounds like those you fought a moment ago. Tend to them and to a guy who tries to attack from the left from the sharply angled corner, but a bigger threat comes in the form of an enemy ahead of you and to the right who will try to fire poisoned darts at you to inflict Toxic. Block those attacks at all costs as you deal with goons, and get behind some wood when you can to eliminate yourself as a target.
Continue onward from there but watch the area ahead of you and below you. It's possible to drop down along the planking toward a monster that looks like a slug with tentacles. You can shoot at it from a distance with arrows if you have them. The beast won't chase you, so it's easy to destroy him from afar. Then you can head forward to find a ladder to a lower ledge. Drop down along those ledges to the area that the slug was guarding to find the Pyromancy: Power Within on the corprse.
In the same general area, you'll also find a number of enemies that will climb around the platforms trying to hurt you. It's worth waiting around for a bit just to let them come to you. You can check corpses for soul-generating items. Near where you killed the slug, the path descends to an area where another of the large goblins with a long club is waiting (like the ones you fought at the entrance to the area from the Depths). You can hit him in the face as he tries to ascend toward you, so it's an easy fight with the advantage definitely in your favor.
Descend along the planking that he was guarding, which itself is positioned against a stone wall, and you'll find the entrance to a cave. It is guarded by another of the faster enemies, but you can easily block its rush and then eliminate it with a few melee attacks. Head into the cave that it was guarding to find another circular chamber like the one that served as the entrance to the area. To the left, you can descend a platform that leads to a lower ledge ahead of a mist-shrouded opening.
Before you pass through the mist-shrouded opening, it's a good idea to equip the Shadow equipment that you found higher in the stage. Its protection against poison will come in handy now, as you're about to venture to an area where you must battle enemies that can inflict poison while you also spend time rolling around in fetid standing water. You should already have the Spider Shield equipped, as well. The combination of all of that will make things go much easier for you.
When you step through the opening, you'll find yourself on some wooden platforms. On a ledge ahead of you, one of the enemies that throws the poisonous daggers is waiting. Hold up your shield and start toward that enemy, though it is in the distance. Don't walk over a platform yourself, but if you move closer the enemy may rush you and fall to his death. Then you can look left, where a torch is burning. You'll find that such torches often serve the purpose of guiding you through a level. Where there is light, you'll usually find other things of interest (as is the case here).
Descend the ladder and a second one just after it, then start forward. A sluggish monster will rush you. Quickly head forward and get to its side, as this monster is capable of breathing fire. A few hacks from the side will disable it. Then you can continue descending and take out another of the monsters before you find yourself looking down a series of wooden platforms. Ahead of you and below, there's another of the dagger-tossing enemies. Be careful to block its attacks. You can possibly take it out with well-aimed arrow shots, or just quickly drop down along the platforms and then head forward to eliminate it.
Once that dagger-tosser is toast, grab the loot from the nearby corpse (you'll find the Wanderer Hood, the Wanderer Coat, the Wanderer Manchette, the Wanderer Boots and the Falchion) and then head back the way you came along the wooden platform. You should find yourself starting to be pestered by insects in this area. Don't try to slash them if you'll have to risk stepping toward the edge of a platform. You may fall off and plunge to your death. A better strategy is to use magic or arrows from a distance.
As you descend through the area from there, the main thing to keep in mind is that there are a lot of the sluggish creatures to deal with. Don't try to rush on your way down. Instead, take your time and look ahead at each ladder. Kill any circling insects, then descend a ladder and take out a slug. Repeat as necessary until you arrive along the bottom of the area.
Now you've found the swamp water. Wading through it saps your defenses and can eventually lead to your character being poisoned. You should leap into the water when you first enter it, then roll and keep moving quickly to minimize the amount of time you have to spend in it. Try to fight any enemies (including more of the slug creatures) on land when possible. You should also be looking toward the right for an archway in the stone. Through that archway, you'll find the region's second bonfire at the base of a stone walkway that ascends toward a tunnel.
Climb that walkway and at the end you'll find a basin. Descend into the basin and open the chest to obtain a precious Dragon Scale. Then backtrack to the area with the standing water and look around for a bit. You can find corpses with items, including the Large Soul of a Proud Knight. There are all kinds of items along this murky bottom.
Straight ahead of the exit to the cave where you found the bonfire, you can roll straight through murky water and work your way around a stone structure to find a waiting corprse. Check there to obtain the Pyromancy: Poison Mist ability. Elsewhere in the vicinity, as you check corpses along the edges of the region and throughout the soupy center, you'll find a couple of the Large Titanite Shard, a Large Soul of a Proud Knight and a bunch of treasure on a corpse guarded by enemies that appear to be giant leeches. In that area, you can find the Tattered Cloth Hood, the Tattered Cloth Robe, the Tattered Cloth Manchette and the Heavy Boots.
Another area of interest, along one edge of the region, is guarded by two of the huge goblins. These fellows carry boulders instead of clubs. They're able to slam down the boulders to inflict severe damage, or to roll them toward you to the same effect. You'll be fighting two at once and there's a lot of swamp water nearby, so you might want to weaken them with some magic or projectiles before you move in to finish off the two goblins. Moving quickly and getting behind them for back stab moves will make the fight go much better. When you defeat them, you will obtain the Great Club, a powerful weapon if you have the strength to carry it.
When you have looted the vicinity, which may require several runs, you can return to the bonfire and to recover and to prepare yourself. Then you should head back into the swamp area and head toward the right.
You'll have to defeat more of the slug enemies if you rested at the bonfire, or if you haven't yet tended to them for some reason. Proceed in that direction, keeping near the wall, and you'll soon reach a waterwheel. You can continue a short distance beyond it to find an area where there is webbing on the side of a hill, with rocks protruding from it like spikes. If you climb up this hill, you'll find the entrance to a new area: Quelaag's Domain.