From Sentinel Comics Wiki
Mini expansions were released one at a time, most often as Stretch rewards on various Kickstarter projects. They are a full deck, available from the store on http://www.sentinelsofthemultiverse.com/store for 5$ each. (4$ for Environments)
The Void Guard is a special mini expansion, containing four hero decks, one for each of The Sentinels characters.
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