If you liked the original King Arthur, you'll probably like this.
If you didn't, there still may be some differences you may enjoy.
Fallen Champions is a linear expansion to the original King Arhur in which you take control of 3 different heroes and their armies and follow their stories in what is essentially a story-based prequel to the events of King Arthur 2. This game uses the exact same engine as the original game and the combat and CYOA book sections play out the exact same, albeit the combat had some improvements.
It is much less of a full game than its predecessor, because it is not a game. This is a stand-alone EXPANSION to the original King Arthur. So it is missing many features from the original game such as kingdom management, but this is intentional as the entire point of this expansion is to be a plot-filler between both King Arthur Games.
If you enjoyed the open world and kingdom management features of King Arthur the most, you may want to skip this. If you don't mind linearity and would like to see the before-events of King Arthur 2, definitely pick this up.