Ascension: Immortal Heroes (2012) is the fourth game in the Ascension series. It is both an expansion to Ascension: Storm of Souls (SOS) and a stand-alone two-player game. It can work as an expansion to any Ascension game, but is best when paired with SOS along with any other sets of your choice.
Immortal Heroes continues with the Fate and Event mechanics from SOS (and includes its own Fanatic card), but also introduces a new mechanic: "soul gems", which when earned allow you to draw a random, one-use card. These cards primarily mirror cards found in the base deck, although they are also pulled from earlier sets.
This is a satisfying mechanic, and it works well with all other sets. This mechanic is notable in that no other sets specifically build on it or contain soul gem cards, so it can become diluted if mixed with multiple sets, but is still fun and useful even if diluted.
This set also adds some other interesting cards, including the Growmite monsters, whose value increases exponentially with each one killed, and some new and useful banishing cards. It also continues the trend of making Mechana constructs less powerful. One result of these changes is that when played as a stand-alone two player game, this set is very recognizably Ascension, but allows for unusual outcomes; super-small hands, big wins by having the right monsters come up, and powerful cards that frustratingly misfire to do nothing. IMHO, it is best played mixed with 3-4 other sets, but it is amusing to play it as a stand-alone set.