Due to the changes of how Windows 8.1 deals with High DPI displays, Microsoft have changed the way the mouse works in Windows 8.1 (also to reduce power usage and battery life on tablets and laptops). This affects users with high DPI mice and mice with polling rates > 125Hz (which is now, almost everyone). Microsoft have released a partial fix to this issue (that fixes cursor movement in mainstream games such as the Call of Duty series), but most games and even the windows desktop (explorer.exe) require a manual fix including a registry tweak for each executable file, execution of a command in CMD Prompt, followed by a restart. However this issue can be fixed by making the cursor in Pillars of Eternity by using DirectInput or Raw Mouse Input (preferred, since Mac and Linux don't use DirectX): Here is some microsoft information on how to use this in your game: http://msdn.microsoft.com/en-us/library/windows/desktop/ms645590(v=vs.85).aspx http://msdn.microsoft.com/en-us/library/windows/desktop/ee418864(v=vs.85).aspx You can also go a step further and give Pillars of Eternity a hardware cursor option (if that isn't already the default case)
Hardware cursors are frame rate independent and make games much more playable on lower end machines during frame rate lag (as the mouse doesn't lag) and reduce cursor latency overall I posted about this earlier but I thought I'd elaborate a bit more on it this time.