The box includes 1 Cthulhu monster board, 1 cardboard figure + stand, 15 Cultist/Temple of Cthulhu double-sided tiles, 12 Madness tokens, 8 Evolution cards for King of Tokyo, 8 Evolution cards for King of New York and 1 rulebook.
The Cultist/Temple tiles and Madness tokens are for play through the utilising of Eight Evolution cards, which are included for both of the base game sets; Cthulhu's Evolution works the same as in the Power Up! expansions for those games, meaning you need the Power Up! Expansion Set for whichever base set you are going to use.
In King of Tokyo, the Cultist tiles are placed next to the board in a face up stack. On a player's turn, when they resolve their dice, if they have 4 of the same faces showing they take a Cultist and put it in front of themself. This tile can be used at any time by discarding it. At this time, the player chooses to either gain 1 health, 1 energy or 1 extra roll.
With King of New York, the Cultist tiles are shuffled in with the building tiles from the base game. Stacks of 4 tiles each are made, building side up. 3 stacks are placed in each of the boroughs of NY just like normal. The Temples are like a standard building in the base game. Once one is destroyed, it is turned over to the Cultist side, which is like a unit from the base game. When a Cultist is destroyed, nothing is gained. Instead, the player takes the Cultist tile, Just like King of Tokyo, they can be discarded to gain either 1 health, 1 energy or 1 extra roll.
Having the mighty powerful, fearsome Cthulhu stomping around extended the replayability of both games; the only thing that would have been better visually would have been having a 3D model of Cthulhu (perhaps in plastic?) instead of the 2D card in a stand-up. Maybe if this expansion is successful it is something IELLO may consider for the future, if it isn't already in the pipeline, 3D monsters!
Madness tokens. These are given to other monsters through playing some of Cthulhu's Evolution cards. At the beginning of a your turn, you must roll as many dice as you have Madness tokens. Once rolled, the dice are set aside and may not be re-rolled. Effectively, they are locked. You then roll the rest of your dice and can set aside or reroll them like normal. When you choose to resolve your dice, the locked dice are resolved as well. To get rid of a Madness token, you use a heart from a rolled die to discard it, instead of gaining 1 health.
Ultimately, Cthulhu does not change the game in any massive way, although it changes it enough so that games are altered if not totally different. As fans of H.P. Lovecraft, and the Cthulhu Mythos in general, it added enough to change the games dynamic for myself and my playing group.