It's the Definitive Edition, which arrived on Steam last year, that will be free from October 18-21, according to Gamespot. Halo Wars was first released on the Xbox 360 in 2009, and came to the PC remastered in 2016, but only bundled with its sequel, Halo Wars 2. The Definitive Edition is the standalone version. [hard to make out if this is PC or XBOX]
They released halo reach on PC and will be releasing the other's in chronological order in 2020. I played the campaign so far its good. Going to be trying multiplayer. Also if you have Xbox Game pass you get it for free.