https://www.overclock3d.net/news/software/halo_wars_definitive_edition_will_be_available_to_play_for_free_this_weekend/1
 
On April 20th 2017, Halo Wars: Definitive Editions released on the Steam storefront, granting access to the game to PC gamers outside of Microsoft's gaming storefront, bringing the game to the masses. GameSpot has reported that Halo Wars: Definitive Edition will become available to play for free on Steam this weekend, starting on October 18th until October 21st, giving PC players the opportunity to try out the game. It is also possible for PC players to blitz through the game's entire campaign over this free period, which takes place several decades before the events of Halo: Combat Evolved.