Prior to answering that question, it's important to understand how each Ascension release is designed.
Each individual Ascension game is referred to as a "set" and, across our first 6 sets, we created a large and small set, which form one "block":
- Chronicle of the Godslayer (Set 1) and Return of the Fallen (Set 2) formed Block 1
- Storm of Souls (Set 3) and Immortal Heroes (Set 4) formed Block 2
- Rise of Vigil (Set 5) and Darkness Unleashed (Set 6) formed Block 3
Each set within a block is developed together and both sets are optimized to be played with one another. However, Ascension is designed in a way that allows you to mix-and-match all of your sets together, so you're never restricted by the block format. All of the core gameplay mechanics remain the same within each set and each of the sets are only differentiated by 1-2 gameplay elements that are unique to that set/block (i.e., Events in Block 2, Energy in Block 3, etc).
In other words, if you'd like to mash up Chronicle of the Godslayer, Rise of Vigil, and Darkness Unleashed, there's nothing stopping you from doing so. The gameplay will be fluid and fun even though those two sets were designed independently.
Moving forward, future Ascension sets will only have one large set box, rather than a pairing of a large and small set. Thankfully, this means you'll get two large sets each year, rather than a large and small set. Realms Unraveled (Set 7) marked the first step in this direction, followed by Dawn of Champions (Set 8), Dreamscape (Set 9), and War of Shadows (Set 10). The individual large sets will build off of the set that preceded it, allowing for a natural pairing between two large sets.
Ascension also includes a wide variety of promo cards. Promo cards are typically bundled in release years that correspond to specific sets. However, much like the sets themselves, promo cards can be mixed-and-matched with any Ascension set. Also, promo cards do not include a defined rarity, however, you can follow these guidelines to determine how many of each promo you should add to each game.