This quest is connected to the main storyline. You activate it when you hear about Shriekers for the first time.
1. Old Ruins - Gareth
2. Sanctuary Of Amadia - Gratiana
3. Fort Joy - Madman's Enclave entrance
4. Fort Joy - Skull Cave entrance
5. Fort Joy - Slane
6. Fort Joy - Alexander
One of Shirekers can be found immediately after entering Old Ruins . The creature guards the entrance which means that you must go around it by climbing a liana. Inside, you meet Gareth. If you manage to save him, he will tell you about a weapon that destroys Shriekers.
If Gareth somehow didn't survive then you must go to Gratiana  in Sanctuary Of Amadia. She also has information about that weapon. Her information are different than Gareth's so even if you defended him you should still talk with the woman.
The next stage is to find that weapon. You have two options.
Choice 1: Purging Wand
The weapon is in Madman's Enclave , inside of a treasury, you can discover it during The Vault Of Braccus Rex quest. The weapon is on one of the plinths.
Choice 2: Radeka's Purging Wand
A witch, called Radeka, has one of these weapons. She lives in Skull Cave . Her cave is full of traps and the witch isn't that cooperative so you will have to fight her. After the fight you can take Purging Wand.
If you chose this option then you can give this weapon to a dragon called Slane . As a thank you, he will be obliged to help you.
Another step is to head to ruins where Alexander is . There, you can meet Ymmit who looks after Shriekers. During the conversation you learn that he is possessed by mysterious powers. Your goal is to destroy all Shriekers that guard these ruins. You can do this by using Purging Wand or by using Purge ability. However, if you decided to help Slane then he will destroy all Shriekers when you get closer to the ruins.