Image found on Wikicommons.
Shriekers are a fascinating creature. They are not dangerous at all. Instead they attract other monsters when they are disturbed. They are a staple of megadungeons. Almost organic traps. Sometimes creatures use them as warning systems. Other use them as a dinner bell.
The shriekers near the Fallow Well are the only living creatures that the wraiths do not touch. The wraiths love the feeling of panic from adventurers and orcs as the shriekers scream.
Some myconids have bred shriekers in several colors. They do this trying to find a harmonic that will summon their demon queen, Zuggtomy. They have succeeded. Red, Red, Green, Blue, Blue and she will appear.
The orcs who now control the Finnius Brewery in Mord Mar have struck a deal with Acidander, the green dragon. Shriekers have been planted in the north alley of the brewery. When the shriekers sound, Acidander comes to their aid. For the grissly price of three warriors. He doesn't eat them, but is instead building his own army.
There is only one passage across the underground river in the Devil's Decision. A sea monster inhabits the waters. It is attracted by the patch of shriekers on the far side. Chances are not everyone will be across when Dark Nessie shows up.