Halo Wars: Definitive Edition is now available as a standalone release on the Windows Store, Xbox Store, and Steam. Originally, Halo Wars: Definitive Edition was only available for anyone who purchased the Ultimate Edition of Halo Wars 2.
Halo Wars itself was originally released in 2009 as the first RTS game in the Halo franchise. The Definitive Edition revamps the experience with remastered graphics, new achievements, and all of the DLC created for the original release. The story itself takes place introduced new characters to the Halo universe, some of whom would later return in Halo Wars 2.
Halo Wars: Definitive Edition is a Play Anywhere title on Windows 10 and Xbox One, meaning you can buy it once and play on both platforms. Keep in mind that the Steam release won't be open to cross-play with the Windows Store and Xbox versions, keeping multiplayer partially segmented. Still, if you're down for grabbing Halo Wars: Definitive Edition, it can be had for a cool $20 at each store now.