Ascension: Return of the Fallen (RotF) is the first expansion for the deck-building game Ascension. While it is possible to play it as a stand-alone 2-player game, it is best played with the base game.
RotF is essentially a new set of cards expanding the base deck: it contains 65 new cards for the Center Deck, along with the basic player cards needed to expand the play from 4 to 6 players. However, it also includes a new mechanic, "Fate", which is simply a random event that triggers for all players when certain cards turn up. Fate events include everyone drawing a card, banishing a card, acquiring a Mystic, and resurrecting a Construct from your discard pile, along with one monster that banishes adjacent cards in the center row. While these are somewhat limited, future expansions will continue to add cards with this mechanic.
Other noteworthy features include one hero and one monster of cost/power 8, both quite overpowered. "Adayu, the Chosen" is a hero that allows you to defeat or acquire any card in the center row for free, and "Samael the Fallen" is a monster that allows you to keep other monsters that you defeat, and play them as heros. While these seem impressive, and under the right conditions they certainly are, they do not actually change game play that much, particularly in shorter games.
The Mechana faction was a strong pathway to success in the base game, and RotF helps balance this by adding in comparatively weak (but still valuable) Mechana cards. Other than this, the factions are pretty well maintained as they were in the base game. This is not to say that the cards are boring or redundant; they are good additions to the game that expand your choices without making any of the base set redundant or underpowered.
Overall, I recommend this expansion to anyone who has and enjoys Ascension, perhaps more than any other. However, it is a smaller one, and if you do not want to buy too many expansions, there is something to be said for skipping this comparatively small set and buying Ascension: Storm of Souls instead.