I've setup NSass from NuGet. It's a managed wrapper around libsass and has a HTTP handler that does on-the-fly compilation of
Found NSass.Wrapper.proxy.dll which cannot exist. Must instead have
NSass.Wrapper.x86.dll and NSass.Wrapper.x64.dll. Check your build
I think, due to the unmanaged library,
AnyCpu cannot be used for the project, instead you must pick a target processing platform.
For me that's 64-bit but IIS Express is a 32-bit process. I recall having similar issues with unmanaged references, bitness and IIS Express (32) vs. production (64) with Oracle libraries.
So when IIS Express tries to load the x64-targeted website assembly, it blows up.
I am now looking into a compile time CSS compilation, rather than runtime, using Web Workbench or something.