The update called Assassin’s Festival will transform the city of Lestalium on August 31 and introduce new quests, themed outfits and more. It is time to send Noctis to jump from roof to roof. It is currently unknown if FFXV-themed content will make an appearance in Assassin’s Creed: Origins.
From IGN’s article:
The companies have announced that on August 30, Final Fantasy XV players will be given an Assassin’s outfit for Noctis if they have collected the Moogle Chocobo Carnival event’s Dream Egg.