Final Fantasy XV: Assassin's Festival was a free downloadable content event for Final Fantasy XV in collaboration with Ubisoft's Assassin's Creed series. It ran between August 31, 2017–January 31, 2018. The event featured gameplay from the Assassin's Creed series (such as stealth and climbing segments), mini-games (such as chocobo racing and "whack-a-cactuar"), and a chance to obtain exclusive items, such as the "Medjay Assassin's Robes" attire for Noctis, Gladiolus, Ignis, and Prompto. If the player had the Dream Egg obtained from the Moogle Chocobo Carnival, the player could unlock the "Master Assassin's Robes" attire for Noctis.

The party arrives to Lestallum during the Assassin's Festival. They meet Holly and Cindy, and change into assassins' robes to blend into the crowd and to take part in the festivities. While Gladiolus and Ignis head off, not being much interested in the assassins, Noctis and Prompto explore the festivities together, as Assassin's Creed is their favorite game series. Within Lestallum, The Assassin is a figure of legend in its history, regarded as a vigilante hero from a far off nation who fought against oppression that once ruled over the city in an age long ago. As they return to the hotel to meet with the others, they learn the Niflheim Empire has made its rounds into Lestallum. Noctis and Prompto shadow Loqi Tummelt, an imperial general they beat back in Duscae, while Ignis and Gladiolus check up on their car. Loqi takes Holly captive when she doesn't let them access to the EXINERIS Industries power plant. As they return to the hotel, Noctis feels his magic powers fading. They find the Leville guarded by Loqi's magitek infantry, but Noctis can warp to the second floor to go unnoticed. The party hatches a plan to save Holly that involves Noctis covertly assassinating MTs around town, as he cannot use the power of kings as long as Niflheim's Wallbreaker Wave is in effect. To get access to the power plant where the Wallbreaker Wave generator is, Noctis and his friends collect keycards from MTs until Loqi himself arrives. Noctis shadows Loqi to the power plant, but after reclaiming the last keycard from him, Loqi escapes and Noctis makes his escape as well, as the magitek infantry begins to attack Lestallum to catch him. Noctis rendezvouses with his friends, and while one of them accompanies Cindy to the power plant to dismantle the Wallbreaker Wave generator, the others defeat Loqi in his magitek armor. Noctis reclaims the power of kings and drives Loqi and his infantry away from Lestallum so the festival can continue.