From Sentinel Comics Wiki
The first Sentinels Expansion to not be 'officially' funded through Kickstarter, it was determined that if the Shattered Timelines Kickstarter reached 150k stretch rewards it would be released early. In one of the last updates before the end of the Kickstarter, >G stated they will release Vengeance early, instead of as 150k stretch reward (The Kickstarter went past 150k in the last 35 hours anyways). It was released February, 2014.
Playtesting began in June 2013, and ended in October 2013.
Often confused as a 'Stand Alone Expansion' this is not entirely true - While this expansion does come with everything needed to play the game by itself, it is best enjoyed with at the very least the core set. Better to consider this a 'Mega Expansion'
Special Vengeance Rule
Because the Vengeful Five use multiple villain decks, if a card affects "The Villain Character Card" the players may pick any one of the villain character cards in play. If a card affects "The Villain Deck", the players may pick any one of the villain decks in play. If a card effects "The Villain Trash", the players may pick any one of the villain trashes in play.