As it is a stand-alone version, you can either use the game alone or you can combine it with the base game. For example, you can use heroes from Mistfall in Mistfall: Heart of Mists and vice versa. Knowing this, you will understand, that a lot of the rules are the same in both games. Therefore, if you are new to Mistfall, perhaps you should read my review about the base game first. However, some new quests and some abilities of the heroes will only work, if you introduce some rules of the new game, too.
Mistfall: Heart of Mists also introduces a new method for the time progress. With the new time charter, the players can regulate the pace of the game. That is something I deeply missed in the base game, because my hero groups often entered the final boss fight unprepared. This does not mean that you can wander from place to place as you want (like in the old Talisman. The game still draws you merciless toward the end. But the time track at least gives you some degree of control of how fast this end will come to you. Since I have the new version, I always use this new element in the game.
Last but not least, Mistfall: Heart of Mists has a new reward system. As you will remember the quality of the rewards in Mistfall were a little bit too random. Sometimes the available rewards were not really useful to take, this also improves with the new reward system.
In the end, Mistfall: Heart of Mists is a worthy successor of Mistfall. The author has improved some of the minor nuisances of the original game and has also added new game elements that perfectly fit to the well-known game flow. The difficult level of Mistfall: Heart of Mists is a little bit easier or better: the players have more influence on the difficult level. In a variant, included in the rules, you can still change the difficult level, so that even veterans have a hard struggle to win the game. Both games of Mistfall are good the way they are, but I personally like the new one slightly better.