In a series of tweets from Merrill's account, the hacker posted images and files related to the project including card art and the main sign-in page for the game. According to "Jason," Supremacy was fully completed but never announced or revealed to the League community. Riot did trademark the Supremacy name last year, lending credence to the hacker's claims.
Marc Merrill eventually recovered control of his account, noting, "We're always working on a variety of new ideas for League and beyond. Lots of experimentation that may never see the light of day."
Make of that what you will.
[Thanks to Dengar for the tip]