This Achievement is for lighting the primal bonfire located in Black Gulch. This area can be reached by falling into the well in Majula while wearing the Silvercat Ring and going all the way to the bottom, then following the path until you reach The Gutter. You'll know you're there when you reach an area that's incredibly dark and basically requires you to keep a torch lit. Keep going and you'll hit an area that's lit more brightly. That's Black Gulch.
The boss himself is known as The Rotten, and is one of the four Old Ones. Your best bet with this guy is to attack from a distance, as he has a grab attack and a smash attack that can both kill you in one or two hits. Thankfully he's not very fast or smart. Just keep whittling away at him (preferably with a little help from someone else), and you'll take him out. The bonfire is just past him, through a small hole in the wall opposite of which you entered his room.