We can start this quest by picking up the Sentient Amulet. This can be found inside a locked chest at the Grymforge.
To reach this chest you need to reach the Adamantine Forge. CLICK HERE for that quest. Once here, from the forge itself you'll see a path going west-southwest. Jump over the gap and follow this to reach a lava pool. Take only one character with you, and the one with the largest jump range (Karlach is probably the one).
Here you'll want to jump across the lava as best you can and make your way northwest. Do be careful because there are parts of the lava pool that explode (you can see the AOE around them) so be very careful. Another thing to worry about is the Lava Elemental walking around. Wait for it to walk away and move to the safe platform where you'll find the locked chest. If your character doesn't have the best abilities to lockpick, pick up the chest and take it with you to where the rest of the party is waiting for you.
As you pick up the amulet a scene will play out where the soul inside it will try to make us laugh. You can give in to the laughter or fight it. Either way the monk will appear and ask you to return him hime. You can ask him about himself and what happened to him, but you can also equip the amulet, put it away, or even discard it. If you ask him about the laughter, he'll tell you about being cursed.
Asking him where he wants to go will give you the info you need: take him to Wyrm's Crossing and to his granddaughter Shirra Clarwen.
The sidequest will continue with no action whatsoever for the remainder of Act 1 and the entirety of Act 2. It's is until Act 3 that we arrive at Rivington.
Once here you'll want to follow the dirt road north from the Rivington waypoint until you reach the Open Hand Temple, which is on the left side. Inside you can find Sister Yannis. You can ask her about Shirra Clarwen and she'll tell you she's down in the crypt under the temple.
Head to the kitchen (northwest part of the temple) and look for an Ornate Wooden Hatch on the ground. Blood should be nearby to indicate its presence.
Upon reaching the cellar, follow the path until you reach a large room with an altar. Go through the western door to find 4 sarcophagus.
Check the plaques on the side of the sarcophagus to find which one is Shirra's (image above). Open it up and see what happens. Yeah, the ghost will possess the body.
Go ahead and talk to it for several dialogue choices. If you happen to have the Arcana Check you can go this route to see what this whole curse entails. Afterwards you get a choice:
- Accept the curse: accepting the curse will force you to pass on some checks. If you don't pass these checks then you'll get -1 or -2 WIS depending on how many checks you passes/missed. On the other hand, if you pass all the checks or just pass the check with the Illithid check you'll then get the Tasha's Hideous Laughter spell.
- Decline the curse: if you happen to decline the curse then you'll have to fight the spirit. It'll reanimate the other 3 corpses in this room, but it's nothing bad really.
And that's about it. Once you complete the above the quest will come to an end.