The NPC for this covenant is Gravelord Nito, who can be encountered as an NPC in the Catacombs.
In order to access him, you must enter an open coffin near the bottom of the catacombs in an area with numerous coffins and a Titanite Demon. You must also possess at least one Eye of Death, three of which can be found on a corpse in the same area beyond the Titanite Demon. The coffin in question is down the hallway on the left. Approach it for an option to "nestle into coffin." Do so to get inside it, then wait several seconds for a cutscene to occur, after which you'll appear in a new area. Make your way over to Nito in his sarcophagus and interact with him to open the covenant menu. Then choose "Join Covenant" and the achievement will unlock.
Important! You can only join and rank up in this covenant prior to defeating Gravelord Nito in the Tomb of Giants.
As always, refer to
http://darksouls.wikidot.com for specific information relating to items or locations.