The Definitive Edition of the original Halo Wars was previously available as a pre-order bonus for the Ultimate Edition of Halo Wars 2. But from April 20 it'll be sold on it's own on Steam and Windows Store, for a reported price of $19.99.
The Windows Store version will support Xbox Play Anywhere. Which means the buyer gets access to both Xbox One and PC versions of the game without any additional costs and some other luxuries. While the Steam version will not support Xbox Play Anywhere; it'll have Steam exclusive features (like Steam Achievements, Cloud saves).
But the sad thing is that the Steam version and Windows Store version will not be compatible with each other. This means, there will be no cross-play between these two.
The official description on the Steam page reads, "Network play for this product is exclusive to Steam. Cross-play with Windows Store versions of the same game is not supported."
Anyway, the Definitive Edition of Halo Wars includes improved graphics, new achievements and all the DLCs released for the original game. Click here to read the official system requirements.