Warhammer 40,000: Rites of War is free from GOG for a short time, until the 19th July.
Store Description (short version):
Inspired by the board and figurine games made popular by Games Workshop, the universe of Warhammer 40,000 is a vast and violent one, rife with strange, alien races, including the Eldar®. The mysterious Eldar are all that remains of a once great and powerful civilization that ruled the galaxy. They travel the stars in enormous spacecraft called Craftworlds, seeking the Maiden Worlds their ancestors created in ancient times.
- Click the link above
- Click “Go to Giveaway”
- At the top of the page will be a Warhammer banner, click the “Get It Free” button on this banner.
- You’ll be asked to log in if you haven’t already. Then the button on the banner will change to a confirmation message when successful.
Aside from this free game, are also several Warhammer discounts and two free DLCs on this page: https://www.gog.com/partner/warhammer.