Enlightenment

Enlightenment is a colorless rare 0-cost skill that reduces the cost of all cards currently in hand by 1 this turn. In a hand of 5 cards averaging 1-2 energy each, Enlightenment provides 2-5 additional energy worth of plays for free. This allows otherwise impossible turns — playing 3-cost cards for 2, or 2-cost cards for 1, enabling more cards per turn than your energy normally allows. Played at the right moment in a big burst turn, it can enable several extra plays.

Enlightenment is best on turns where you have several high-cost cards in hand that you want to play all at once. The free 0-cost Enlightenment itself plus reduced costs on 4-5 other cards can enable massive turns. It's particularly strong in boss fights where you need to maximize output in key burst windows. Upgraded, Enlightenment's cost reduction becomes permanent for the combat rather than just this turn, dramatically improving its value — each card in hand becomes permanently 1 cheaper, compounding over every subsequent draw of those cards.