It seems something's wrong with the Knight of Tir-Cendelius, rather than the Shriekers. I could manage to finish the initial conversation with him, but the game would still crash as he walked away. Any party member being within his visual radius while not stealthed will crash the game.
The only workaround I've found is to kill him. Once you've killed a second Shrieker, he will move down to the clearing by the Shriekers. You will want to kill all of the Shriekers first, while staying outside of his visual radius (which means even when you're behind him the game would still crash). You can make an Invisibillity potion (Chanterelle + Empty Potion Bottle) to help with this. Then, have a sumnon attack the knight to initiate a fight.
Debugging the error dump files with Visual Studio gave me this
"Unhandled exception at 0x00007FFADDFEF08E (ucrtbase.dll) in EoCApp.exe.2532.dmp: Fatal program exit requested."
Sadly I have no idea what exactly this means or how to fix it.
