Killer Instinct has been gifted to various Xbox Live members by Microsoft who meet certain criteria.
According to Microsoft, the game is being gifted to members with high Gamerscores, and years active through Xbox Live.
"You're a fan of us and we're a fan of yours," reads the email sent to users. "Thank you for your dedication to Xbox. We want to show you our appreciation so we're giving you Killer Instinct.
"We've unlocked all characters so you can hit the ground running. Watch your inbox on 22 November for instructions on how to claim your free gift."
Killer Instinct will release alongside Xbox One on November 22.