Although the Killer Instinct reboot is a free download on day one, you’ll only have access to one character, the rest are paid downloadable content.
Microsoft decided to reward their fans by giving out the full version for free in an email newsletter. This has surfaced from Neogaf, where multiple users are reporting getting the same email, which you can see below:
So this is pretty cool, although it seems like there isn’t a set parameter to go by when figuring out if you’ll get the game for free or not. Perhaps it’s a combination of luck and having an Xbox Live subscription for multiple years?