The new series entry was revealed on Saturday with a debut trailer carrying an ESRB AO for Adults Only logo.
An Xbox store listing for the game, which has since been updated, also initially carried an AO rating, with content descriptors including real gambling.
Assassin's Creed Mirage: Cinematic World Premiere
“Following the announcement of Assassin’s Creed Mirage during Ubisoft Forward, some store pages mistakenly displayed the game for preorders with an Adults Only ESRB rating and are being fixed,” the company said.
“While Assassin’s Creed Mirage is still pending rating, Ubisoft wants to reassure players that no real gambling or lootboxes are present in the game.”
Set for release in 2023, Mirage takes place in ninth-century Baghdad at the peak of its Golden Age and is described as a “special tribute to the first Assassin’s Creed”.
With gameplay focused on parkour, stealth and assassinations, Mirage marks a return to the action-adventure style of Assassin’s Creed games released prior to RPG-style series entries Origins, Odyssey and Valhalla.
Development is being led by Ubisoft Bordeaux, the studio behind Assassin’s Creed Valhalla’s Wrath of the Druids expansion.
Mirage was one of six upcoming Assassin’s Creed games discussed during a Ubisoft product showcase on Saturday.
The next flagship series entries after Mirage, codenamed Red and Hexe, will be released as part of the new Assassin’s Creed Infinity franchise hub.
Red, which is being made by Odyssey studio Ubisoft Quebec and has the same creative director, is set in feudal Japan.
Ubisoft is also working on a standalone Assassin’s Creed multiplayer game and two mobile series entries.