Story related. Cannot be missed. On NG, I strongly recommend triggering te Maneater Mildred invasion before entering Quelaag's boss battle (must be human).
Quelaag is a simple fight. She has a blind spot to her left where none of her attacks will hit you. Hug that spot and be sure to jump out of the way when she spits the fire at you! Also, when you see her character model bunch up, roll back 3 times so you can get away from the AoE attack. This attack will devastate you if you have lower VIT. Once you kill her, onto the little stair case and pull the lever to ring the bell and to get your achievement!