There has been some slight confusion on whether or not Halo Wars: Definitive Edition would be available to purchase separately or if it even was possible to buy it without Halo Wars 2.
The people at Xbox saw the confusion and came to the rescue with an update on their Support page. There are two ways of getting Halo Wars: Definitive Edition, either you pre-order Halo Wars 2 and receive a code in your Message tab on your Xbox or in you Xbox App on Windows 10, iOS, and Android when the Definitive Edition is made available on December 20, 2016, or you purchase it separately. Unfortunately, players will have to wait until later in 2017 for that option.
"How do I get Halo Wars: Definitive Edition?
When you pre-order Halo Wars 2, you’ll receive a code for Halo Wars: Definitive Edition when the game is made available early on December 20, 2016. The game won’t be available to purchase separately until later in 2017."
This should clear some of the confusion regarding how to get Halo Wars: Definitive Edition, either get it as a code when pre-ordering Halo Wars 2 or purchase it separately later in 2017, the exact date for when it becomes available as a standalone purchase is yet to be announced.
Halo Wars 2, on the other hand, is scheduled for release on February 21, 2017, and those who pre-order it get four days of early access. Halo Wars 2 will also be a part of Microsoft's Play Anywhere program so buying in on one platform grants access to it on both a Windows 10 PC and on Xbox One.
