As detailed in a series of since-deleted tweets (via Kotaku), the hacker claimed Riot Games completed development on a card game called League of Legends Supremacy, but "buried" it for reasons unknown.
"Riot doesn't want you to play this game," the hacker said.
League of Legends Supremacy was reportedly a standalone game, though no further details are available. Riot Games trademarked the name "League of Legends Supremacy" in May 2012.
The hacker threatened to release a 200mb file containing all League of Legends Supremacy images, but later said, "I have decided to give Marc his Twitter back."
"Yup, someone got onto my Twitter account. Yup, someone shared some old screens from one of the many prototypes we've experimented with," Merrill said, after he regained control of his Twitter account.
"We're always working on a variety of new ideas for League & beyond," he added. "Lots of experimentation that may never see the light of day :)"
Blizzard Entertainment recently announced its own card game, HearthStone: Heroes of Warcraft. The game is currently in beta.