Chaos Witch Queelag

Quelaag is one of the three Daughters of Chaos who were not entirely caught by the Bed of Chaos's power: her lower body was morphed into a demonic spider, but her upped body remains as it was and she retains her consciousness. She appears as a boss in Quelaag's Domain

Location
Quelaag's Domain, below Blighttown.

Background
Quelaag once lived with her mother - Witch of Izalith - and her two sisters in the ancient city of Izalith. After the Flame of Chaos incident, all but her sister Quelana were transformed into nefarious demons. She and her sister only known as the fair lady escaped the worst of it, having their lower torsos bound to giant spiders. Fleeing their deranged, demonic kin, they climbed to the base of blight town and Quelaag fashioned themselves a home.

Important notes

 * Quelaag is completely immune to fire damage. Do not attempt to use fire enchanted weapons or pyromancies in this fight. (Actually she isn't completely immune, I just beat her and took away a chunk of health using FireBall pyromancy)
 * Quelaag uses a combination of fire and physical attacks.
 * Her sword attacks are surprisingly weak and can be blocked like the attacks of most normal foes. Her attacks deal fire damage, so a shield that has good fire resistance (Black Knight Shield, Dragon Crest Shield) can be a lifesaver in this fight.
 * The spider-mouth in the front can and will spew out a variety of lava attacks. All of these leave a varying (depending on the attack) amount of lava globs on the floor. These do a lot of damage if stepped into, as well as stunning you for a moment. The globs disappear after a while, fading out as they do so.
 * If you summon Maneater Mildred and she delivers the final blow to the boss killing it, you will still get the demons soul, however you will not get the 20,000 souls.

Spider

 * Lava stream. Slow buildup, long-lasting. Quelaag's spider-mouth spews a solid stream of lava left to right. Telegraphed by the witch pushing the spiders head down for a while.
 * Lava glob. Quick attack. The spider-mouth lowers and spews out a small blob of lava right under it. Telegraphed by the witch dipping the spiders head quickly.
 * Lava spit. Quick attack. Small blob of lava shot a distance away from the spider's mouth, followed by Quelaag leaping to the side. While up close, this attack will look similar to the longer lava-spewing attack, but differs in that the glob is not followed by a solid stream of lava, but rather thinner, seethrough lava that doesn't settle on the floor.
 * Side stomp. Long buildup. The spider raises its legs on one side of the body, and then stomps them all down on one side of Quelaag's body for high damage. Even if not hit by the stomp itself, small shockwaves erupt from the impact points, doing fire damage and making it hard to punish the attack if it misses. Telegraphed by the spider leaning back on its hind legs for a while.
 * Lava spray. Quick attack. Spider will light up and send fireballs in all directions.
 * Lava sideswipe. Attacks may be followed up by a quick, solid stream of lava in a quartercircle to the side of Quelaag's body. Seems to be in response to the player being on Quelaag's side.
 * Lava pounce. Used when the player is at medium-long range. Without a doubt Quelaag's most dangerous attack. Spider lowers its body and then jumps quickly forward toward the player, spewing a quarter-circle of lava that lands in front of where it lands. If the spider lands on you, it does high damage.

Quelaag
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 * Flame Aura - Slow buildup. Slumps forward over the spider's head, charging up a high damage AoE attack. Usually follows a Lava stream attack. This is often the time when the player is attacking because the spider is spewing fire, so it is worthwhile to save a bit of energy for blocking the AoE blast or rolling away.
 * Firesword swipe. Quick buildup. Raises her sword, striking horizontally in a circular pattern. Can easy be avoided by circle strafing or walking backwards, thus saving energy for a shield block.
 * Firesword stab. Medium buildup. Quelaag raises her sword, stabbing straight ahead with huge fire damage, can be avoided by circle strafing or rolling backwards.
 * Sword combo (3 moves). Performs two Firesword swipes followed by a Firesword stab. Similarly to other enemies Quelaag will stop the combo if the player is too far away.

Getting next to the spider-head
Just mind your footing and draw Quelaag forward and away from her lava sprays. Watch the woman for cues and not the spider.

One the fight begins, target Quelaag and head straight for her. She wields a flame sword that she will swing in a familiar right, left, thrust combo. Get as close as you can and she will swing over your head, missing every swing. All of the swings seem to have a blind spot on the left side of Quelaag. You can take a free shot after every miss. Once the combo is over she will usually hop back and retreat. From here she may begin any of the other attacks. Be on your guard for a pounce, as it is the most difficult to avoid. Once she's started another attack, approach from either side and get between the spider's head and foreleg again to get some strikes in. Occasionally after taking damage Quelaag will pass out and use Flame Aura to repel you. Keep your shield up and weather through it.

Keeping the spider at distance
The safest way of doing this fight is to stay away from her until her spider starts doing its left to right lava spewing attack, it takes quite a long time for it to do this so you should be able to get some solid hits in. Do not stay within range too long however because of the possibility of her AoE. Always keep your guard up when moving away from her if you do see that she's slumped over, the range on the AoE is very high and it deals enormous amounts of damage if unblocked.

Her sword attacks are fairly easy to avoid, just move backwards while shielding yourself and the horizontal slashes won't hit you. The stab may hit you however since the range on it is very long, but if you are guarding it should not do a whole lot of damage.

Arguably the most dangerous part of this fight is when the spider decides to leap towards you. Unless you are very far away from her you risk being hit by the lava spewed in front of the spider. Rolling underneath the spider is also an option, should you find yourself to be too close to her. Note that if you don't move too far away from her she won't jump at you. In the end, how to react to this ability requires your own judgement.

Wedged spider
As soon as the fight begins run to the door up the small bit of stairs in the back of the room, the right corner is just far enough to cause the boss to become stuck trying to run towards the player making the fight horribly easy. That is unless she decides to fill said corner with lava, then you will die.

Up close and personal
If you keep Quelaag very close and keep strafing around towards the back area of the spider, most of the attacks will miss you. The spider legs are very thin and will never crush or damage you unless it is doing the foot stomp which is slow and easy to spot (small circle of damage as well) The spider spewing is easy to avoid because it is slow and telegraphed and being very close, you can be around the back attacking before the spew is finished.

The room is big enough to turn the spider in a tight circle and avoid any lasting fire blobs. The spider jump attack does not happen because you are too close all of the time. However, you do need to keep an eye on nearby blobs of lava, especially towards the end of the fight.

The sword swipes are frontal damage only and the large fire thrust with the sword attack is quick but always in a straight line and easy to avoid because you are turning in a circle. The only problem is the AoE attack because it does high damage and if your shield is down or low on energy you can be killed. You can always recover from this by drinking the flask while the spider is spewing fire. It shouldn't take too many hits if you're two-handing your weapon and swinging every chance you got. Just remember to avoid the explosion attack and you will achieve victory.

Stunlock (requires phantom/s)
It is possible to stunlock Quelaag with the Gargoyle's Halberd. You need to two-hand the weapon and time your R-1 so that she is locked in place and cannot cast spells. You will want to go up in front of her and attack her human torso. Attacking Quelaag's human body also deals extra damage. While she is stunlocked have your phantoms just attack her nonstop till she dies. Very simple method as long as you have enough stamina regen. (Only tested in first play through). Also confirmed to work with the Great Scythe.