Buy Assassin's Creed® Mirage | Xbox
Experience the story of Basim, a cunning street thief with nightmarish visions, seeking answers and justice as he navigates the bustling streets of ninth-century Baghdad. Through a mysterious, ancient organization known as the Hidden Ones, he will become a deadly Master Assassin and change his...
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Given the rest of the content warnings are pretty standard for Mature rated titles, the AO rating seems to come from the Real Gambling warning, which is defined by the ESRB as "Player can gamble, including betting or wagering real cash or currency", as opposed to Simulated Gambling which is defined by "Player can gamble without betting or wagering real cash or currency". Going by their Rating guide, Simulated Gambling is allowed from rating T and up whereas Real Gambling is only allowed for a rating of AO.
Mirage is still some ways out, so it is likely that this rating is a provisional one of what the publisher expects the game to be rated as, but I don't think I have even heard of an AO rated game in many years.