This bonfire is accessible after finding the King's Ring (Which is found at the end of the Undead Crypt in the room where where you fight Vendrick; it can be grabbed without fighting/defeating him. You will also need the ashen mist heart which is given to you by the Ancient Dragon at the end of the Dragon Shrine) From the Cardinal Tower bonfire, go down the ladder in the room with the bonfire. Then turn left at the bottom of the ladder and defeat the 2 hollow soldiers and go right across the bridge that leads to the other side of the chamber. Go right at the end of the bridge and through the door, there will be a path to the left which leads to a King's Door. Behind the door is a bonfire. This is the bonfire that will be used for the farm.
The farm will the the "Memory of Jeigh" which is the memory that is accessed by walking up to the dead giant in the same area that the Place Unbeknownst bonfire is. Enter the memory and you will appear in a small hallway with a blocked exit in front of you and a mist door. Enter the mist and you will be on the top of a castle wall, with giants and soldiers fighting on top along with consistent bombardments by large fire bombs. The boss is at the end of this wall. As soon as you enter the mist, begin to run forward, sticking to the right side of of the path. once you make it past the second giant, the head of a large statue will roll down the path, killing the giants (and you if you do not hide on the right side of the path.) Now, all that is ahead of you is the Giant Lord boss fight. Don't bother trying to block his attacks, they will all 1-shot you through any shield, just roll and avoid all of them (the rolling timing can be difficult to get at first, but once you get it down, you can easily kill this boss). Now, there is a platform to the left that you can walk up on to, and the Giant Lord wont follow. If you walk up on here, he will be forced to only use his overhead smash and horizontal slash attacks, which are both relatively easy to to dodge. The core of the fight is basically just dodge his attack, cast a spell, and then dodge the next attack. Rinse, Repeat and the boss will go down shortly. The bonfire ascetic can be found by reentering the memory and going up on the first platform to the left, it will be on a corpse. You can then homeward bone/aged feather back to the bonfire and burn your newly acquired ascetic and repeat the entire process for even more souls.