Official Riftbound ruling

What card does Astral Heron’s triggered ability discount?

It discounts the next card played (added to the chain and queued for finalization) this turn after the ability resolves.

Astral Heron has received an erratum to update its wording: “When you play your first card each turn, if I'm at a battlefield, the next card you play this turn costs 2 Energy2 EnergyAny RuneAny RuneAny RuneAny Rune less.”

Riot's question and answer, word for word, from theVendetta Rules FAQ and Clarifications

The card

  • Astral Heron
    Astral Heron

    Unit

    When you play your first card each turn, if I'm at a battlefield, the next card you play this turn costs 2 Energy2 EnergyAny RuneAny RuneAny RuneAny Rune less.