Chaos Witch Quelaag
Skills: Fire slash, Fire stab*, Lava spit*, Lava leap, Ground stab, Fire spray,  Fire explosion*
Times died on 1st play through:5-ish
Ugh. After the bout with the Stray demon, its time to go back to where we left off in "The Depths" and go into the dreadful "Blighttown", one of the worst maps in the game. From there, venture to "Quelaag's Domain" and meet the owner herself. Since "Queelag's Domain" is a very small area in its own, she is obviously the only boss in the area.
Prepare to be burned!
Well Quelaag actually has some nice fan art of her looking quite hot, despite her being on top of a grotesque spider demon. Anyhow, she is definitely a boss that is stronger than many others in the game so she in no way should be underestimated. She does some very high damage and is able to infest the entire area in lava, making further dodging a complete pain. And without a fire resistant shield or fire resistant equipment, fighting her is going to be rather difficult.
Fire Slash- She swings that deadly flaming sword around. Nothing special, it can be blocked and dodged easily.
Fire Stab*- This one will hurt and has a deceptively long range. She stabs her sword in front of her, covering quite some distance and heavily damaging you if you don't block. Even if you block, you will take some good damage and stamina reduction.
Lava Spit*- This attack itself is very predictable and easy to look out for, but she will use it over and over. Soon, the entire map will be flooded in lava and you will find yourself having little room to move around. Not that she is most vulnerable during this attack. 

Lava Leap- She jumps at you and starts spitting lava in your face. Dodging this is the way to go, since having to block lava right after she jumps you is usually a bad idea.

Ground Stab- Her weakest attack. She just uses those spider legs to slam the floor around her. If you are hit you will definitely get hurt, but it is very easy to avoid.

Lava Spray- She just sprays blobs of lava around her. Another attack that you won't need to worry about too much.

Fire Explosion*- This one is just crazy. Her human body leans forward and she lets out an explosive fume that covers a huge radius that deals crazy damage and cannot be blocked.

She has a lot of attacks, and because of that, you have quite a few things to look out for. Quelaag herself doesn't give a lot of openings for you to attack her, and you only have about 1 or 2 opportunities to properly hit her out of the many attacks she will use on you. As I said, it is highly recommended to bring along fire resistant equipment to further reduce the damage she will be doing.

Her first attack is fire slash, and this is just her flailing that sword around in front of her in a 180 degrees arc. It is easily dodged by rolling back, or can be blocked, but by blocking you take minor damage because of the fire attribute it possess. It isn't anything too dangerous, but you should still exercise caution. Fire stab is her only dangerous sword attack, and that is because of its range. Though it is only a stab, the range is very deceptive and even blocking it can hurt quite a fair bit. Needless to say, if you take the full stab, its going to hurt like balls. Just stay as far away from her as possible if you want to be safe. Be careful of surrounding lava!

Who knew they would have such sexy fan art? They are spider people!

Lava spit is actually the only attack which leaves her vulnerable, but I marked it as an attack to look out for. Why? While she will stand there and let you hit her, the map will quickly fill with lava, and soon, it will become very hard to dodge. And yes, while the lava will disappear after some time, it still stays there for quite some time and will become a nuisance while you try to dodge her other attacks. If you accidentally roll into some lava while dodging, it will still hit like a truck!

Lava leap will cause Quelaag to leap to you at about mid range, and then she will spew lava in your face after that. If you block the whole thing, things might get ugly, so its best to just dodge it completely. Still, if you screw up and get hit by the leap, the follow up lava spit and really hurt you bad. Ground stab is her weakest attack, as all she does is just stab her legs into the ground, and can be easily avoided by just backing off. Same to lava spray, it isn't very dangerous as all it does is send out small blobs of lava around her.

Her trump card is lava explosion, and this one hits like a boss. She will lean her body forward and the spider will let loose some lethal explosive fumes off a huge radius. This is unblockable and does some insane damage. There is just no way around this one, you have to just pray that you are at a respectable distance and roll away! If you are like, in her face when she starts this up, you are going to take the full brunt of the attack.
There is no way around it, she is quite a threat. The only attacks that are safe for you to hit her are the lava spit, ground stab and lava spray. Not so much on the lava spray if things get out of hand, but the other 2 are perfectly safe. However, there is the issue with the lava. At any time when you dodge and lava is around, you are quite f**ked. Also if she decides to attack ON the lava, you cannot retaliate. This is a very troublesome issue with Quelaag.
Talented artists, beautiful picture.
That is for melee, but there are other ways around it of course. Ranged combat via archery can be rather difficult, as you won't have much time to cancel animations if she goes for a leap or starts to swipe her sword in your face. You have to be constantly on the move and look out for lava, thus archery isn't recommended. You can try sorcery, but it isn't as safe either since at this point, you won't have a lot of the more powerful and hard hitting spells like moving soul arrow.

20000 souls for this spider beauty is quite a good reward. You even get her soul, which can be used to craft the almighty chaos blade, one of the best DEX weapons in the game. Congratulations! You have defeated the guardian of the second bell! Ring it and its time to continue your journey! Until next time!