Illustrated by Kudos Productions
7When you play your first card each turn, if I'm at a battlefield, the next card you play this turn costs 
2 Energy
Any RuneAny Rune less.
Errata
Updated text
When you play your first card each turn, if I'm at a battlefield, the next card you play this turn costs
2 EnergyAny RuneAny Rune less.Printed text
When you play your first card each turn, if I'm at a battlefield,
yournext card costs2 EnergyAny RuneAny Rune less.
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Official rulings
· Riot's answers, word for wordWhat 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 EnergyAny RuneAny Rune less.”