This Achievement is for the Covenant of the Fittest. To join them you must pray in front of the Victor's Stone. This can be found by going straight from the hut where the game begins until you reach Majula. Now head to the left until you see a few buildings. Go past the buildings until you reach a cliff's edge with a path heading up the side. The stone can be found at the end of the cliff.
The Covenant of the Fittest makes the game harder, so think carefully before joining. Keep in mind that you don't actually have to join this group to earn the Achievement. The game will ask you a few times if you're really sure that you want to join this Covenant. The Achievement should pop right before you have to fully commit to it, and which point you can back out if you so choose.