To enter the room and gain access to the bonfire and Spider Witch, you will need to tell Eingyi (the creature that blocks the entrance) that you are interested in joining (note that this does not count as committing to the covenant). Led by the ' Daughter of Chaos', it located behind a false wall on the first floor of the room behind the boss of Blighttown ( Quelaag).
Chaos Servant is a Covenant in Dark Souls.
