Unity Technologies will offer support for the DirectX 12 graphics API, included as part of Windows 10, for PC game developers first when they use the Unity Engine to make their games. DirectX 12 support for Unity Engine games made for smartphones and the Xbox One will come afterwards.

In a blog post, Unity stated:

Currently our plan is to release DirectX 12 support early in the Unity 5 cycle (well in time for the release of Windows 10), and to target standalone builds and Windows Store Apps first. Then we'll follow up with Windows Phone and Xbox One support, aligning with the plans Microsoft has for their platforms.

The blog said that the Unity Engine team has been working for a number of months to get the engine working with DirectX 12 and currently it passes 95% of its graphics tests. It added:

Passing all the tests is only the first step. Once we are happy with the implementation feature-wise we shall move on to the new features of Direct3D 12 and invest heavily in optimization. There is ongoing work to allow our renderer to better use multiple cores which will apply directly to Direct3D 12 and hopefully provide the promised performance improvement. Right now, it's too early to discuss performance due to the alpha state of Windows 10 and DirectX 12 drivers, however we are happy with the numbers we're seeing.

DirectX 12 will be included as part of Windows 10 but it will not be made available for people who stick with Windows 7 and Windows 8.1 on their PC.

Source: Unity