This is a very missable achievement on the way to a boss fight, so be prepared for some spoilers in this solution.
So, first thing you want to do is find this place, called the Darkroot Garden
. If you've found Blacksmith Andrei
in Undead Parish
, you're making good progress so far. In his room is a staircase that leads downstairs, and in that room is a Titanite Demon
. You're free to just run past him, but if you're seeing this guy, you're going the right way. Behind him is a door that leads to the Gardens, and here's where the fun begins.
Now, in order to progress the story you have to enter this area and kill Great Grey Wolf Sif
, so chances are you'll walk right past this covenant holder. Go here to this map
. See that #1 at the bottom of it? That's where you start. See the #16 guarded by the Knights, Bandit, Cleric, Hunter, Thief, and Sorceror? That's where you join the covenant.
Regardless of which route you took to get there, you'll encounter this cat in that room.
Talk to it, and all you have to do it choose "Enter Covenant" to get the achievement. You don't even need to actually enter, just talking about it works.