Go to The Caverns and talk to an NPC named Mody, play his game and find him twice.
Dig up the mound that is revealed near where you found Mody the second time.
Follow the path down and talk to Withermoore, the tall petrified skeleton.
You'll need a Strength based character to pull the spear out of Withermoore, about 12 or so should do the trick. If you do not have the required Strength you can attack and destroy the spear.
Agree to help free his soul and he'll tell you about a secret trap door at the shrine in Fort Joy's camp.
Go to the shrine to the seven and activate the trap door from the dialogue, you'll end up in an underground area where you'll find a waypoint shrine, speak to it to unlock teleporting to shrines. There will also be a locked door with the lever to open it on the left of it (facing it from where you came in), you can also pick the lock.
You'll now be in the The Holding Cells of Fort Joy's castle, pick the lock at the top of the stairs or defeat the Magister Hound Master and his followers to gain the key to unlock it.
This'll lead you to the 2nd floor of the castle, don't worry the silent monks are harmless unless you attack them. Follow the hall to the staircase opposite of where you came up from and you'll see another staircase heading up, go to the little corner next to it and a lever should be revealed to you that will take you to an underground chamber. (You can put the vases on the gas to stop it.)
At the end of that hallway you'll find multiple soul jars; you want to break the one that says Withermoore the Supplicant, all of the others spawn traps or a horde of skeletons that will prove challenging to lower level heroes.
Return to Withermoore and grab his belt from the corpse.
Later during the quest The Gargoyles Maze/A Fate Worse Than Death you will meet Withermoore one last time as he helps you through a source powered gate, upon interaction with the adjacent glowing relic (however, the character interacting with the relic must have no source points or Withermoore will not appear). This will complete his journal entry.
1,400 Experience for smashing Withermoore's Soul Jar, setting him free.
Lord Withermoore will come back to you one last time to unlock the barrier door inside Braccus Rex fortress and thus mark the quest as complete.
Alternatively you can absorb his soul and dooming it in the process for 1,000 Experience. If the player doesn't have any source points open it makes doing this moot. The player gains less experience and gains nothing in exchange.