Spoilers below! This covenant achievement is only available after a certain point in the story.
After defeating Quelaag and ringing the second bell you will have access to a room beneath the bell lever. In this room there are spider webs and eggs covering the walls, one of those walls is breakable. Slash your way through and you will be in a hallway with a man with eggs all over him blocking the way.
Just talk to him (select "Yes" when prompted) and he will move.
The person to talk to is just beyond him and to the right... you can't miss her... she's pretty obvious.
You don't have to enter the covenant, just select "Enter Covenant" from the dialog options with her and then you can make your final decision in the next prompt ("YES" or "NO")- regardless you will have the achievement.