X-Cost

X-Cost is a card mechanic where the card's Energy cost is denoted as X rather than a fixed number. When you play an X-Cost card, it spends all of your remaining Energy, and the card's effect scales based on how much Energy was spent. For example, a card that deals 3 damage per Energy spent would deal 9 damage if you had 3 Energy remaining.

X-Cost cards are most powerful later in fights when you have already played your other cards and have substantial Energy remaining. They synergize extremely well with Energy-gaining effects like Energized, and are especially devastating in decks that can generate large amounts of Energy per turn. Notable X-Cost cards include Whirlwind (Ironclad), Skewer (Silent), All for One (Defect), and Alpha/Judgment (Watcher). A common strategic consideration is whether to play an X-Cost card early or hold it for a large turn.