Dead Kings is DLC for Assassin's Creed: Unity that is available for free to anyone who purchased the base game. It was meant to be a commercial expansion, but Ubisoft decided to release it for free to compensate for a number of bugs in the original game. People who already bought Assassin's Creed: Unity - Season Pass could choose a full game from a selection of Ubisoft titles. For the PC version is added to the base game automatically as a patch through Steam or Uplay, while for the consoles it needs to be downloaded as a separate item.
The story takes place after the main game and works as the 13th memory sequence which is longer than any of the memory sequences from the Unity main storyline. Arno takes a coach ride to a smaller city of Saint Dennis (at the time known as Franciade) and leaves the Paris. There he meets his old friend De Sade and ends up on a quest to find the mysterious item hidden deep in the necropolis, all the while fighting the Raiders and racing with Napoleon to the same prize.
Game introduces Raiders as a new faction which are mostly encountered in the catacombs and caves, while you'll still deal with soldiers and bandits while traversing the city streets and sneaking into protected areas marked with either blue or red. The map is split into two segments, underground and town which consists of three map synchronization points. Raiders will often come in larger groups, but they have their commanders which are tougher opponents, but once defeated, the rest of them scatters around pleading for their life. New gameplay element is the lantern you can use to light your way through the caves and catacombs, and scatter around bats, spiders and other bugs which would otherwise hinder your progress. You can refill your lantern by dipping it into oil jars you can find all in the underground areas.
This DLC also features many new weapons and outfits. New swords, new spears, new heavy weapons, and an entirely new type of weapons known as guillotine guns which serve as an axe and a historical version of a grenade launcher. There are several new outfits which include Guard of Franciade outfit, Raider outfit, Napoleon's Artillery outfit, and Iron Mask outfit.