Quelaag is a tough early-game encounter and one you'll face after the infamous Blighttown.
Quelaag boss strategy
Quelaag is a very difficult boss fight, she spews the arena full of lava, has devastating AoE attacks, and can hunt you down at a moment's notice. With multiple lava attacks that stay on the ground for a period of time, you will definitely want to be keeping an eye on the state of the arena at any given moment of time. Furthermore, in addition to this, Quelaag has large-range with her sword, so be sure to not get caught up in her flurry of attacks, which can easily stunlock you.
Right, you will want to keep your shield up for this fight, and find one that has decent fire resistance, as Quelaag's sword attacks deal fire damage in addition to physical. Be sure to keep moving in this fight to avoid her pouncing attack, which will almost always kill you. We will be breaking down her sword attacks first, of which she has 3.
There is a sword combo, which does 2 swipes followed by a stab, the best way to avoid this is to roll around to her hind legs and start attacking. There is a single swipe that can be avoided by simply walking backwards diagonally, as well as a sword stab, which will deal a huge amount of fire damage. Again use the strategy of strafing diagonally backwards in order to get out of the way of these.
The moment you see Quelaag slump forward, you will want to get away from here as soon as possible. She's charging a flaming AoE attack that can be a one-hit-kill for low Vitality players. This is often followed up by an additional Lava Spew, so do watch out for her attacks.
You are also able to summon Maneater Mildred into this fight if you are in Human Form, and it is a great way of drawing aggro away from you in order to focus on attacks to the rear. Do be aware however that Maneater Mildred is able to die pretty quickly in this fight, if she manages to stay stood in the Lava that's being spewed. (Thanks, AI)
In her rushdown form in Spider Mode, you will want to keep your distance away from her unblockables and watch out for the windup on her lava spewing attacks, which should be relatively easy to dodge. From here, you will also have a strange opening to the left of her, when she is using her sword attacks roll towards her front leg and counterattack. Then, you will be able to do a bit more damage.
Once you've managed to get Quelaag's health down a fair bit, you will notice that she will begin to attack and spew lava and her AoE more often, be sure to read her obvious tells to guess how to react. Really, with the sword attacks, you can just strafe diagonally backwards, it's as simple as that when it comes to close range, as long as you keep your eyes on the arena and placement of the lava in front of you, then you might be able to do a bit more damage.
With these tips in mind, you should have everything you need to take this spider-witch down. Again, keep an eye out for that pesky AoE as it becomes more regular throughout the fight, and there is a summon available to you, should you need it.
If you're looking for more help, our Dark Souls walkthrough and guide can help with all other areas of the game, including the dreaded Taurus Demon, Capra Demon, Ornstein and Smough bosses. Meanwhile, we have explainers on how to access DLC Artorias of the Abyss, the best gift and best class in Dark Souls, Dark Souls' best weapons, Dark Souls Covenants, Dark Souls Estus Flasks locations, Dark Souls Titanite Slab locations and NPC quests including Lautrec of Carim.
After defeating Quelaag, you will get access to the Second Bell of awakening in Quelaag's Domain.
Before you go, there's a hidden bonfire here to find. Head downstairs, and strike the back wall for it to disappear. Beyond here are some friendly characters you can talk to, as well as the bonfire. It's worth visiting this now as you'll need to come back later.
Once you're done, it's time to head to Sen's Fortress.