Madness

Madness is a colorless uncommon skill that permanently reduces the cost of a random card in your hand to 0 for this combat, then Exhausts. At 1 energy, it converts a random card in hand to a 0-cost play for the rest of the fight. If it targets a 3-cost card, it effectively provides +2 net energy over one activation. Over a long boss fight, having a permanently free high-cost card compounds enormously. The randomness means you can't choose the target, but in a carefully curated hand, most outcomes are valuable.

Madness is best played when your hand contains high-cost cards you want to make permanent 0-cost plays. Targeting a 3-cost card like Sands of Time makes that card free for the entire fight — a massive value gain. The random selection means sometimes it will target a 0-cost card already, generating no value, so it performs better in hands full of 2-3 cost cards. Upgraded, Madness reduces the cost of 2 random cards instead of 1, roughly doubling the energy savings and making it far more consistently impactful.