Released as a major free update for existing owners, the brought massive graphical overhauls and gameplay refinements to the entire campaign. Key updates include:

New specialized lighting effects that make the underground facility feel more atmospheric and immersive. How to Access the Game

While many sites claim to offer a "Free Download" for v1.5, it is important to distinguish between the legacy mod and the modern game:

This is the version that contains the Definitive Edition content, the full Xen campaign, and official multiplayer. Supporting the developers here ensures you get the latest security patches and bug fixes.