Praise the sun. Our Dark Souls Walkthrough provides tips for surviving the adventure, a guide to the entire world, and hints for slaying bosses.
Kiln of the First Flame
This is the final area in the game and can only be accessed once you have obtained the Lordvessel and have defeated the bosses that lurk at the end of the Tomb of Giants, The Abyss, Lost Izaleth and the Crystal Cave. When that is the case, you can head down to Firelink Altar and place the souls in the vessel (which you will have previously placed on the altar in order to gain access to the required four boss areas) to open a doorway.
As you start through the area, you'll descend some wide stairs where spirits pass over ahead of you. The spirits don't harm you, even if you walk directly through them, so just keep descending.
You'll come out in an area filled with what look like dunes of ash. Though you can walk up and along hills to the left and right, for the most part all that you need to do is descend. There are no items to worry about collecting just yet. As you descend, you'll run into the occasional enemy. There's really only one type of enemy in this area, besides the boss. It's a knight with terrific defense and the ability to hit you harder than really seems fair. Not only that, but once you're hit a time or two, you'll probably keep being hit because your stamina will be gone and you can't even get up without meeting a sword thrust to the gut from your foe.
There are two knights to worry about as you descend through the ash dunes. Fortunately, there's plenty of time to heal after both encounters if necessary.
When you have descended to the base of that area, you'll see narrow ledges ahead of you. These cross a wide gap, and there are more of the knights along the ledge. Your best bet here, if you have the ability, is to hit the knights with magic from a distance as they approach. That way, you won't have to worry about them knocking you into the pit.
Near where you meet the first knight in this treacherous area, you can head right along the ledge to find a corpse dangling from the edge. Whoever managed to get this far before you apparently fell short just a bit ahead of the goal. You can loot the Black Knight Helm, Black Knight Armor, Black Knight Gauntlets and Black Knight Leggings from that unfortunate someone, then head back to the intersection and continue in the direction you found the knight.
Along that route, you'll soon ascend and find a moss-lined stone path that leads in a gradual arc toward the right as it descends toward a mist-shrouded door. Along this route, you'll find two more of the knights that you've already faced, spaced far enough apart that you only have to fight one of them at a time. Continue relying on magic to eliminate them safely from a distance if you like, then continue onward to the mist-shrouded door.
When you're ready, step through it to face the game's final boss.
Gwyn, Lord of Cinder
As boss battles go, this one can prove rather anti-climactic if by now you're good at parrying and counter-attacking. Those are the two skills that will serve you well as you battle Gwyn.
When you first appear in the area, Gwyn will be standing at a distance. When he spots you, he'll start running toward you. You should rush to meet him, shield ready, but it's worth noting that throughout this fight, Gwyn is capable of hitting you so hard that even if your stamina is maxed, you'll lose a lot of it and possibly take some damage by follow-up strikes. Gwyn can often hit two or three times in a row before he is temporarily exhausted and easy to hit.
Dodge the attacks that you can, then, and move in to get in a blow or two of your own. You want to be careful about staying too close, though, because Gwyn can skewer you, lift you into the air and then fling you back. That move drains a lot of stamina, of course, and it also gives Gwyn time to recover so that before you can stand up, he'll rush you and leap into the air then try to bring his sword down in a powerful arc that has a good chance of proving fatal. Make sure that you roll out of the way and then counter-attack in such instances.
Don't just use all of your openings to attack, either. If Gwyn has your health in a bad place, it's perfectly acceptable to gulp down potions before your foe recovers. Remember that once you start the animation once, you can also guzzle subsequent potions without waiting as long for them to take effect. You want to keep your life meter in good shape so that a surprise move by your opponent doesn't spell your end.
If you have a good weapon, a good rating for your strength and decent stamina, this fight will go very well for you. Just keep everything in mind that you've learned up to this point, apply it and victory will soon be yours!
After you defeat the boss, you'll receive 50,000 souls and the Soul of Gwyn, Lord of Cinder. Then you'll be able to choose the ending you want based on your next actions. Head to the bonfire that appears at the center of a nearby ring of stones if you want the noble ending, or try to leave the cavern without using a bonfire if your wish is to be a scoundrel. After the closing credits conclude, you'll immediately begin the game's New Game+ mode. Enjoy!