This achievement is called "Sanguinary Covenant".
The solution contains as few spoilers as possible but read with caution.
After the Executioner's Chariot boss fight in the Undead Purgatory go up the stairs at the end of the long, curved hallway. The opening is at the end opposite from where you came in.
After going up the stairs you'll come to a room with a bonfire and an NPC. You need speak to that NPC a few times. He'll start asking you if you like blood and so forth. Answer "Yes" to his questions until it asks you on the screen if you want to join the covenant.
You don't actually have to join the covenant to get the achievement. If you wait on the message confirming your decision the achievement will pop. Same thing if you're already in a covenant as well.
Edit: An Aegean Cat brought up a point that I completely forgot about.
You need to have at least one token of spite. You can get one for invading and defeating someone, which you can do by using a cracked red eye orb.
Edit 2: Super Aw3some received his Token of Spite from defeating an invading NPC in Huntsman's Copse.
Edit 3 per Giff the Stiff:
A Token of Spite can also be found: in the bottom of the pit in Majula. However this requires you to purchase Laddersmith Gilligan's most expensive ladder, the Silvercat Ring (unless you have enough HP to survive the falls, doubt it) or a Fall Control spell. (removing items/equipment helps reduce fall damage as well) You'll need to fall down onto the bottom most ledge which leads to a section filled with scaffolding and ladder upon ladder. At the very bottom is a chest containing the Token of Spite. This path leads to The Gutter and allows you to skip the Grave of Saints area; a popular pvp area.