Praise the sun. Our Dark Souls Walkthrough provides tips for surviving the adventure, a guide to the entire world, and hints for slaying bosses.
Tomb of Giants
You can enter the Tomb of Giants by way of the Catacombs area. After you defeat Pinwheel, you'll climb out of his tomb and appear on a trail with lights positioned along it. As you walk along the trail delineated by the lights, you'll see the text appear on-screen to indicate that you've entered the new area. Note that you should not bother attempting to clear this area until after you have claimed the Lordvessel from Anor Lando, or you'll just be blocked from proceeding once you reach the bottom of a decidedly inconvenient dungeon. It's also not a bad idea to bring along a divine melee weapon, to make the area boss fight go more smoothly.
When you're ready to head into the tomb, move slowly as you follow the trail marked by the lights. They don't give off enough light to make it clear where the trail begins and ends and there are a lot of dangerous falls that you can take as you descend through this area. If you have the Sunlight Maggot helmet that gives off light (found near the end of the Lost Izaleth region beneath the Demon Ruins), you'll have much easier time in this stage because you'll see better and because you won't have to give up your shield to use the lantern that can be found partway through the stage. We'll give instructions here that assume you haven't found that helmet, so that you're covered either way.
Follow a straight line from the first light to the second one, then start for the light to the right (but move slowly to make sure that you step onto the walkway that bridges the gap in that direction), then head carefully forward with the wall to your right. You'll come up against one of the giant skeletons, so defeat it with fire attacks if possible so that you don't have to dance around too much on the uncertain footing.
When the skeleton is dead, continue down the trail with the wall still to your right as a guideline. You'll see a corpse to the lower left with a tempting glowing item, but you're not safe going directly for it because there's a pit. Instead, continue descending along the wall and toward the next light.
Here, the lights point to the top of large fallen pieces of stone. You can climb on those to slide down to a lower ledge, then do the same with another of the stone slides. At the base, an archer from the side will try pelting you with magic arrows, while to the right you can start forward to find yourself battling two giant skeletons. They're a real threat if they get you together, but you can hit them with fire magic (while hopefully avoiding any arrow projectiles) to make things go easier. Then continue descending carefully, a few steps at a time as you proceed past the area where the skeletons were positioned. Slide along another of the huge stone slides (this one isn't marked by lights, alas).
You should soon reach the edge of a ledge overlooking some lava. There's the top of a ladder here. Descend the ladder, then head left along the ledge to find a bonfire. Go ahead and light it if you're sure you want to proceed through this area.
From the bonfire, you're now safer to explore the upper areas without much fear of losing progress, if you're interested in what items you may have missed during your cautious descent. If you head back up the ladder and veer left, you'll find a stranger. Talk to him and he'll ask if you are a cleric. Answer “no” to his question (you'll miss out on stuff if you go with a different answer). Then head toward the nearby light and a scene will follow.
You'll be pushed over the ledge that you were exploring and will fall to the ground. Your friend on the high ledge will gloat about the treasure he'll be able to collect from your corpse. There are other corpses already nearby and you can investigate them to find the Skull Lantern, which you can equip and use like a shield to see your surrounding area. You'll also find two of the Large Soul of a Proud Knight, but that's less exciting.
With your lantern equipped and in use, head around the area where you've fallen. Near a stone column, you should find an NPC. You can talk to her to learn that the members of her escort have been turned Hollow. If you proceed past where you meet her, you'll be attacked by the two warriors in question, Vince and Nico. You'll do best if you can draw back from a distance and use fire magic or firebombs to eliminate them, since they have strong armor and fierce attacks that make it difficult to fight the two of them. When you defeat them, you can talk to the girl again and you will receive Replenishment, a helpful miracle that regenerates HP over time.
Once that task is tended to, you can keep looking around the area to find a White Titanite Chunk on a corpse at the top of a path that leads up along one wall. Note that in this area, you'll encounter a new type of enemy: skeletons that form walking totem poles. They're strong against physical attacks, but go down almost instantly to fire attacks.
When you're done exploring the area in the hole, you can head out along the route that Vince and Nico guarded. That trail leads to more of the towering totem skull enemies. Defeat them in the same manner as you did their predecessors, then find a ladder at the end of the corridor. Climb it to its top and it will seem that you've found a dead end. However, you can roll through the wall to arrive on a high ledge.
Be careful on that ledge. There's an archer firing powerful arrows at you from somewhere to the left. For now, you should head to the right to return to the bonfire. If you head left instead, you'll find an oversized stone tomb. You can drop down through a hole to land in a lower area, but it's full of huge skeletons. Defeat them and you can obtain the Soul of a Brave Warrior and the Large Divine Ember by checking corpses in the area.
If you're interested in returning to the start of the area, this is the route to take. Further up the trail that leads left from the coffin, you can find some more large skeletons out in the open. Defeat a guy with a sword and an archer, then climb a ladder to a higher ledge. From the upper ledge, you can drop down to the left to pick up the Large Soul of a Proud Knight. There are some enemies waiting on the high ledge, as well. They're more of the large skeletons, so act lively as you arrive or they'll take you out before you even figure out that they're hiding in the darkness ahead of you. You can find another Large Soul of a Proud Knight on a corpse that they were guarding. In the back corner, hard to see against the wall, there are metal rungs on a ladder. You can climb up along that route to reach the start of the area, when it's time to leave (you can also drop down on a ledge and find three of the Eye of Death from the top of the ladders and the Large Soul of a Proud Knight on an even higher ledge, which is always nice).
Assuming that you return to the bonfire rather than leaving the stage, you should next head back up to where the man pushed you down the hole. If you head right and forward from there, now using the lantern that you obtained, you'll find a large skeleton lizard crawling about. Hit it with fire attacks from a distance, or careful sword strikes at closer range, then advance past it to find a mist-shrouded opening. Pass through it.
On the other side of that opening, you'll find some real trouble. There are a variety of threats.
First, if you head toward the left you'll find another of the bone lizards. It's joined by an archer not far beyond that, so you need to be careful to conserve your magic and use it to the most positive effect. After you clear out that upper area and grab the Soul of a Brave Warrior that rests on the corpse the archer was guarding, you have a possible detour worth taking. You can drop off the end of the ledge and if you land properly, you'll find yourself on a ledge with a corpse ahead of you. Check the corpse to find the Covetous Silver Serpent Ring, which will cause the skull totems to appear around you. Hack them apart and then return through the cave to the approximate area where you dropped down to find the ring. Head around and you'll find a path that descends toward a lower tier. Here, you'll need to battle yet another of the bone lizards and also another of the giant skeletons.
If you manage to defeat that threat, you can continue roaming around and you'll find another ledge that descends even further before coming to a second bonfire. You're almost to the lowest point of the stage now.
Head back up from the bonfire and explore the upper ledges again. You'll need to defeat any enemies that were revived when you rested, as usual, so that means at least a bone dragon and a large skeleton. Climb up a slope from there and veer left so that you're following a narrow ledge with a wall to your left. You'll soon meet with a golden knight. If you're carrying around a lantern to see, remember that you don't have a shield. He'll just repeatedly lance you and the only way to survive is to back away and toss powerful fire magic at him while you avoid falling from the narrow ledge where the fight begins. If you get past him, you can check a corpse on a ledge to find the Effigy Shield.
Backtrack to the area's second bonfire. You're ready to descend through the last leg of the stage now, having collected any worthwhile treasure. Start back up the hill from where you found the bonfire and pay attention to the path. It branches left and descends along that route, below where you can fight the bone lizard and archer. Descend along that path and you'll soon find a recess on the right with a corpse in it. You can check the corpse for a White Titanite Chunk.
Continue descending along the trail from there and you'll soon find three more of the bone lizards. They're off to the left side and the path you need descends to the right, so you may prefer just to run past them and keep going. As you descend, watch for the path to double back on itself; you can descend through an opening there to find the Lost Soul of a Brave Warrior. Then continue your descent.
Soon you'll reach the top of a ladder. Climb down it, then descend along a cliff-side trail. It leads eventually to an opening bathed in orange light. The opening is sealed unless you have already defeated the boss enemies in the Abyss. You won't be able to proceed any further here unless that is the case.
Assuming that all is well, take a moment to change up your equipment to best handle the coming area (you no longer need to use a lantern, thankfully) and proceed through the opening.
As you start through the opening, you'll find yourself on a long path that descends through rock. There are places where the ledge falls away into an abyss, so you need to progress carefully. Along the trail, a couple of totem towers and some more skeletons will greet you, but ahead of that there's one of the rare crystal lizards that you can kill if you're fast enough.
Once you beat those enemies and continue along the ledge, you'll come to the entrance to a larger cave area. There will be a bunch of skeletons lined up on the edge of the ledge ahead of you. Below, miniature versions of the Pinwheel foe you fought in the Catacombs are positioned. There are around five of them throughout the area. The closest will be able to target you as you stand along the ledge. Their magic can break through most shields, so be careful.
Drop down from that first ledge and work around toward the left to easily take out the closest pinwheel magic user. You want to stay on the upper ledges of this room as long as possible while you destroy the various casters. Their magic shouldn't pose much of a threat, since you can duck behind stone walls or columns and then rush them as the sorcerers prepare additional magic blasts.
As you work to eliminate the sorcerers, look around the cave to find a mist-shrouded doorway. It's largely obscured by bones but isn't far from where you first enter the area, on an upper ledge. You can simply rush for that door if you'd like, but then you'd miss some items. On the plus side, you'd also miss spending much time fighting the miniature skeletons that spawn endlessly in the lower portion of this room. They're weak monsters but they rush you and jump on you, quickly bumping up your Toxic meter. If you get hit by a swarm of them, you'll fall to that status ailment, which is why it's a good idea to take out the sorcerers first.
Once you're able to explore, you'll still have to run from the skeletons or hit them with magic or a quick weapon, but you'll more easily find the area loot: two of the Soul of a Hero, the White Titanite Chunk and the White Titanite Slab. The items are often hidden around the back side of stone columns, so you'll have to look carefully while keeping an eye on the miniature skeletons so they don't build up enough numbers to overwhelm you. The slab is easiest to find if when you first enter the room with the sorcerers, you keep dropping down along the left side of ledges and continue in that direction until you find the prize at the end of a narrow corridor.
When you have finished exploring and looting, head to the mist-shrouded door and pass through it to find the area boss.
As you appear in the cave where you'll battle Nito, you're on an upper level and will have to drop forward off the ledge. That can cause you to take some severe damage, so be ready to gulp down some estus flasks after Nito's introductory animation concludes.
Nito looks imposing but isn't actually a huge threat. He's a floating mask of blackness and skeletons, armed with a massive sword. The bigger problem as you battle is that he is joined by a bunch of standard and slightly larger skeletons. If you try to battle Nito without dealing with them first, you'll have to work extra hard to avoid having most of your attacks interrupted by slashes from the side that can quickly drain your life meter.
The best trick is to let the skeletons come to you. Kill them with a divine weapon so that they don't respawn, and keep an eye on Nito to avoid his huge sword thrust. If you let him close in on you, then you can move in close and hack at him with your weapon or toss powerful fire magic at him (it works wonders, as you might expect of an undead foe) without worrying about causing more skeletons to leap to their master's aid.
As long as you're good at avoiding the infrequent attacks Nito levels at you with his sword, and as long as you're not letting his skeleton crew pick you apart, this fight is one of the easiest of the late-game battles to win.
When you defeat the boss, you will receive the Lord Soul. Then you can light a nearby bonfire to find the means to warp out of the cave.