The NPC for this achievement only appears at one place in the game : Memory of Vammar.
In order to get there you need to obtain the "Ashen Mist Heart", it's part of the main quest.
You can refer to this achievement on how to obtain it *As for now there is no solution yet, but it will come in time*
You also have to kill the Giant Lord in Memory of Jeigh, the tree is found behind the door requiring the King's ring. If you don't know where the door/tree is, at the 2nd bonfire, go down, take the small bridge to the other side of the fiery pit, go right and through the door and take a left after the door. You should now be facing the door requiring the King's ring, equip it, go through the door, rest at the new bonfire you've just discovered if you need to and you should now see the tree in front of you and interact with it.
The memory has a limited amount of time (I'd say ~5 minutes). There's 2 Giants, the Giant Lord, a bunch of explosion (none of the in the area of the boss) and a huge rolling stone head that comes of the statue. Run as far as you until you see the head rolling, you can go to safety on the left and avoid the head. The explosion and the head should take care of the giants and any small enemies. Go around the head (the right) and kill the boss. The boss is similar to the Last Giant you killed a the start of the game except he has more life, hits harder, has sword while having two arms (compared to Last Giant with either 2 arms or 1 arm + 1 arm as a weapon). When you're done, to exit simply go up the step and interact with the man like statue on the ground, once to get a soul, twice to exit.
Now it's time to go to Memory of Vammar. How to get there? Teleport to the 2nd fire of the forest of the fallen giant, run all the way to be turtle shell guy, but instead of going that way, take a right until you get to the giant like tree. When you examine the tree you should be teleported in the zone.
When you're inside Memory of Vammar move forward until you reach a room with a door you can actually open, don't open the door yet. To the right of the room, there's a man. Keep talking to him and the achievement will eventually pop.