Cross the threshold before paying for the payoff
Herald of Spring · Concentrate · Master Yi, Unstoppable
Herald's play trigger gives two XP. Let it resolve before paying for a card that becomes cheaper at the new level. Concentrate costs five Energy normally, three at six XP, and one at eleven. Unstoppable costs ten Energy and two Calm Power at three XP, eight and one Calm at six, then six Energy and no Power at eleven. Those reductions replace one another rather than adding together. If Herald takes you from nine XP to eleven, Herald itself has already entered exhausted. The new ready-entry benefit applies to the next unit, not retroactively to him.
