Memento Mori is here! As advertised by Zynga, the banner for this states "Prepare for the greatest fight yet.". Momento Mori probably has some thing to do with Fight, who knows what Zynga is planning to do, time will tell. For the detailed guide, do visit this page: The Arena: In Game Sneak Peek.
Each time you access the game, the Memento Mori screen will show up. You can ask for Power Cards via that screen.