For the quest "The Shriekers", refer here.
To kill Shriekers, use a Purging Wand from Act 1 if you still have one. If not, use Source Vampirism. You get this spell as you progress the main quest line automatically. Because of Source Vampirism's low range, use Stealth to get in range of the Shrieker and use the spell from Stealth.
Note: Fix the lay-out of the page and add additional (lore) information on Shriekers.