This Achievement is for joining the Rat King's covenant. This can be found by entering the giant well in the middle of Majula. You'll need to purchase the Silvercat Ring off of the cat in Majula, which reduces the damage inflicted by falling. Equip the ring, then drop down the wooden planks until you see a stone landing jutting out partway down (do not go all the way, that's too far).
You'll find a bonfire once you reach the landing. Follow the path from there until you reach another bonfire, followed shortly by a boss fight against an infinite amount of rats. The trick here is to kill the one unique rat, which will cause the rest to run away. He'll drop his soul as usual, along with a rat tail.
The rat king himself is in the room just after the boss fight. Speak to him a couple of times with a rat tail in your possession, and he'll offer to let you join his covenant, earning you the Crest of the Rat ring. This works in a similar fashion to the Bell Keepers. You equip the ring, and anytime another player enters the rat king's domain, they'll be summoned to your world as a grey invader. Kill them to reap the rewards. For starters, the enemies in the area will no longer aggro on you while you wear the ring. Additionally, you'll earn a Pharros' Lockstone for each enemy you defeat.
Oh, and try to make sure you don't make a jackass out of yourself by hitting the rat king when you first see him. Oops.