Ascension: Darkness Unleashed is both a standalone game for two players and an expansion that can be combined with Ascension: Rise of Vigil or other Ascension games.

Ascension: Darkness Unleashed features "Dark Energy Shards", a new take on the "Energy Shard" Treasure cards introduced in Ascension: Rise of Vigil. Like an "Energy Shard", a "Dark Energy Shard" allows a player to draw a new card while providing one energy to power up Energize cards. In addition, the Fate effect allows each player to banish a card in his discard pile whenever a "Dark Energy Shard" is placed in the center row.

This set also features new Energize cards, including Heroes, Constructs and Monsters that transform into new beings or more powerful versions of their original selves. If you would play a card that has a Transform ability and you've met the Energize threshold on that card (by having played a certain amount of energy that turn), then you ignore all the effects on the original card and place the transformed card in your hand, which can now be played this turn and on future turns for the remainder of the game. (Ascension: Darkness Unleashed contains both double-sided transform cards for those who play with card sleeves and single-sided transform cards for those who don't. In the former case, you remove the card from the sleeve, flip it to the transformed side, then resleeve it; in the latter, you remove the original card from the game and bring in the transformed card.)

Other cards – such as the monster "Erabus, the Exiled" – are transformed and added to your hand when you defeat them in the center row. At the end of the game, you score Honor for whichever version of the card is currently active in your deck.