This quest chain has four quests, and I believe is required for you to get the Ending the Drought achievement.
The area you need to get to is actually up on the stonework of Lantern Hook itself. If you are coming from the Porticulum, head SouthWest to where the Wardstone is. Continue on, moving around the outside of Lantern Hook until you come to the end where you climb up to your left, and then continue to climb NorthEast up the rock. You should be able to see the tree by now, but be careful not to fall off the edge of the ledges that hold it.
You will notice that the tree has a hole in it's trunk, as if a large branch had been broken off revealing the hollow tree inside. The ledge with the quest item on it is INSIDE the tunnel the tree holds, not the ledges around it.
There is no quest marker on pile of bones, so you cannot see it on the mini map, there is just a pile of bones (called Moldering Bones)that sparkle a little. I uploaded a screenshot here. The ledge is roughly SouthWest of inside the hole entrance. I found putting a marker on the mini map for that direction helped me quickly get the direction right.
I spent a huge amount of time trying to get this quest item. I failed (and died) a lot. The best bet is to get to the tree, dismount and climb up roughly above the hole, then mount up and then edge forward until you get into the hole. Someone on the forum here said that walking forward into the hole worked, but I found walking without a mount just didn't give the distance to get to the ledge. Once I tried it with the mount, I got lucky and landed on the ledge.
Could you describe how to get up there? I am having trouble finding a route.
I found the tree with the hole... from the Ancient Wardstone go up and left to reach the tree, but could see nothing from there that looked like a quest starter. The ledge directly below the tree is Felix's "Court".
Jump into the hole in the tree. Its a long thin pipe that leads to the pool under LH. Midway down there is a small ledge with a sparkly (interactive) object. You must be at least level 36 to use the object.
Hitting the ledge is an operation in frustration. If you miss, you either die when you hit a wall join from falling damage, or you fall into the well and have to swim out and ride around again. If you happen to hit the ledge and die, use a soulwalk to res in place and use the object.