A fatal directX error has occurred (100000)

While DirectX 11 is backwards compatible with prior DirectX versions, it may not handle version-specific commands like the actual version of DirectX the game is made for. To circumvent this, it is agreed with the other suggestions of downloading and installing the 9.0c redistributable, which will not modify or wipe or interfere with DirectX11 in any way shape or form. DirectX 9.0 will be used if an application if specifically written for it, which FF14 was written for 9.0c. Wherever you are getting your information from is completely inaccurate, or if the information stems from fears of what will happen to your PC there is no need to worry here.