This Achievement is for joining the the Bell Keeper's covenant. There are two ways to join, and finding the first one requires a little effort on your part. You'll come across a bonfire after defeating the three sentinel bosses in Lost Bastille. Right next to that is a ladder. Go down it (watch out for the dog) and use a Pharroh's Lockstone on the little face in one of the walls. This will highlight a spot on another wall, which you can then knock down to reveal a secret passage. Note that this wall cannot be broken down unless you've used the lockstone. The new passage leads to a door and the Belfry Luna area. Just after entering you'll see a tiny man sitting on top of something to your right. Talk to him a few times and he'll eventually offer you the chance to join the Bell Keeper's.
The second place you can join is the Belfry Sol area of Iron Keep. Towards the end of the level, you'll find yourself in a big open room with lava at the bottom and some platforms suspended over it. A knight will probably be firing at you with arrows as soon as you enter the room. Go through the nearby door and use a Pharros' Lockstone. Now strike the marked wall and go up the ladder. You should see a bonfire once you reach the top. Go up the ladder next to it to reach the Belfry Sol area, along with another little man you can speak with to join the covenant.
This covenant operates in a similar fashion to the Forest Keeper's from the original Dark Souls. You'll be given a ring, and whenever other players enter the bell tower area, you'll be summoned to kill them. Defeating players in this way nets you titanite chunks, so it definitely has its advantages.