Killer Instinct and all of its downloadable content is currently free on Xbox Live for Gold subscribers. What does this have to do with PC owners? Well, since Killer Instinct has a cross-buy feature, it means all content purchased on Live goes for Windows 10 Live as well.
In short, Windows 10 Live owners can pick up Killer Instinct and all of the Season One downloadable content for free.
Killer Instinct is a remake of the original Rare fighting game released in arcades and the Super NES back in 1994. The game was sort of a hybrid of Street Fighter and Mortal Kombat, featuring the former's gameplay and the latter's Finish Him! mechanic, called "No Mercy" in the game. The remake earned mostly positive reviews as one of the best Rare remakes for Microsoft's Xbox One. (The original game appeared as an Xbox Live Arcade title.)
Gamers with an Xbox Live Gold subscription can buy the entire Season One content. The game itself is free-to-play. The Season 2 pack remains premium priced, while Season 3 is expected in March 2016. The Windows 10 version is due sometime in 2016.