This Achievement is for lighting the Brightstone primal bonfire. This is located in the Brightstone Cove Tseldora area, which can be found by progressing through the Shaded Woods, past the scorpion boss there and into Doors of Pharos area. Go up the stairs on the left here and follow the path straight to reach Brightstone Cove.
As usual, the primal bonfire is guarded by one of the Old Ones. In this case, the Duke's Dear Freja. Freja is an enormous spider that oddly enough can shoot a laser out of one of its two mouths (one on each end). The trick to beating her is to quickly run around to her opposite side as soon you enter her area. Smack her on the mouth located on her ass a couple of times, then quickly circle around to her "front" side and do the same thing. Repeat this process until she falls over dead. The key is to not stay on either end of her body for too long.
With Freja gone you can proceed through the nearby hole in the wall. There will be a single guy you have to kill in a place called the Duke's Private Chambers. Keep going from from there to the room with the bonfire. Be careful, though, as another enemy is waiting for you in there, though he's easy enough to beat by circling around and backstabbing. He also drops some pretty nice armor.