This area is tacked on to Lothric Castle, and is very small (so this will be a correspondingly small guide). This area also has the boss Oceiros, the Consumed King at the other end. There are still a few big, hard hitting knights around, presumably to guard Oceiros. (Nobody seems to mind that he's taken over a whole garden.) Still, there are a couple of bits and pieces to find, and going through here is mandatory if you want to get to the Untended Graves or Archdragon Peak, two more entirely optional areas that have a lot of useful items and are worth investigating.
Start at the Dancer of the Boreal Valley bonfire and go up the ladder into Lothric Castle. Take the left hand path and you'll come out at the top of a narrow staircase on the side of the castle. Immediately on your right is a door that becomes a shortcut; we'll open that up in a minute. For now head down the stairs, past or through the knight, and onto the lift at the end. This will take you down into the garden, the dark, misty areas of which will stack poison onto you, so keep that in mind.
The only bonfire you can light here is the Oceiros, the Consumed King one, which, as you've guessed, requires that you finish the boss fight. Confusingly, when you light this bonfire it appears on the High Wall of Lothric list for fast travel. So that's helpful.
The Garden has clearly seen better days. There are four squiggly abyss beast slug monsters roaming about, as well as a knight or two, so unlocking the shortcut first is a good move. Head along the right side of the swamp, around the round, raised platform rather than up on to it, and follow the right wall until you come to a doorway. There'll be an abyss beast somewhere on your left and the doorway has moss and ivy growing up onto it. In here is your lift, which takes you up into a new room, split over two levels. At the back of the top level is the door you passed earlier. Open it up to get the shortcut to the garden from the Dancer bonfire; this also opens up the quickest run through to the boss fight.
Take the lift that goes down into the garden – the first one, mind, with a knight guarding it, not the shortcut one – and about half way down jump off it, onto a higher floor. You'll be able to see where easily. Walk to the right end of the walkway you're on and the shard is on a body here.
Without this you won't be able to get to Archdragon Peak, but to get it you have to kill Oceiros, the Consumed King. After you've beaten him you can open the doors at the back of his arena and run down a corridor and round to a combined tomb/treasure room. There's a dead body in front of the Lordvessel in the middle of the room. This is where you get the Path of the Dragon gesture, although it could also be the Generic Meditation Pose Gesture.You need to perform it next to the statue where you found the Dragon Torso Stone in the Irithyll Dungeon to get to Archdragon Peak.
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