Hey there Slackmackan, thanks for this feeback, we greatly appreciate it!
Can you attach a dxdiag from your system, please? To get a DxDiag press Windows Key + R, then type in dxdiag and hit run. On the menu that comes up, press the "Save All Information" button and save out the file. A DxDiag will list most of your computer's information so we can diagnose the problem properly.
Also, can you give us some information on what you were doing before engaging Medreth? Like transitioning to different zones, save/loading, previous combat encounters.
Thanks!