For those unfamiliar with Ascension at all, the game is a fantastically fun deck-building game that moves quickly and is easy to learn. As this is a review for a later version of the game, if you would like to read more about how the basic game plays I would recommend checking out some of the reviews for the original release, Ascension: Chronicle of the Godslayer.
I already own both Ascension Chronicle Of The Godslayer and Ascension: Return Of The Fallen Expansion, so while I realize this can be played by itself, I have only played with the other cards mixed in. However, the additions this expansion makes are pretty cool. There are two main changes: Events and Trophy Monsters. Events are a card type that get moved to a separate place on the game board when they are flipped up in the central row, and remain there until a new event shows up. There are five events total, one for each faction + one for the monsters. They always add a global effect, an ability which each player can use in a turn, and some other cards get a bonus depending on which event is out. The downside to these is that since I have both previous Ascension games mixed in, 5 events in a ~250 card deck means they don't come up very often. If you are playing this game as a standalone I think you'll see them be a bit more game-altering. The second major change to the game is Trophies. These are an ability on monster cards that allow you to take the monster after you defeat, and use the trophy ability whenever you want, but only once. For example, if a trophy event on a monster is "draw a card", you get that monster card once you defeat it and you can draw a card with it right away or save it for another turn. Overall, a very nice addition. There is also a new "Always available" monster called the Fanatic which has a trophy effect(you can only have one of them at a time, though).
One other new feature they made is the option of a solitaire game where you play against the strongest monster included in the set, Nemesis. I don't really think I'd enjoy playing the game solo though so I am not super familiar with the rules for the solitaire version.