A variety of issues may be caused by corrupted game files, e.g. immortal enemies, crashes, glitched graphics or inability to launch the game. If you have encountered such an issue, please verify the integrity of game cache and load a game save file created before this issue occurred (if possible).
GOG.com: 1. Launch GOG GALAXY and log in with your GOG.com account. 2. Choose the game from 'Owned games' tab. 3. Click 'Customize' button (to the right of "Play" button) > Manage installation > Verify / Repair
Steam: 1. From the Library section, right-click on the game and select Properties from the menu. 2. Select the "Local files" tab and click the "Verify integrity of game cache..." button. 3. Steam will verify the game's files - this process may take several minutes.