Found behind an illusionary wall below the area where you ring the bell, in Quelaag's Domain is a small area with a Bonfire and two NPCs. The first character is Eingyi. The second is Quelaag's sister, the Fair Lady.

Just like Quelaag, the Fair Lady is half giant spider demon and the upper part of a female. Unlike Quelaag however, the Fair Lady will not attack you.

Talking to the Fair Lady will grant you an option to join the Chaos Servant Covenant. You will need to be wearing the Old Witch's Ring before any other dialogue makes sense however.

Fair Lady, Quelaag's Sister, leader of the Chaos Servant Covenant and mute until you wear the Old Witch's Ring.

credit: juras.imgur.com

• Leader of the Chaos Servant Covenant.
• Mute unless you wear Old Witch's Ring.
• Drops a Fire Keeper Soul if killed.