Heart of the Forest
From Archon Arcana
Heart of the Forest • Untamed • Artifact • Rare • Each player cannot forge keys while they have more forged keys than their opponent. • Artist: Alyn Spiller • Card Number: AoA (355)
Commentary
If an ability saying keys cannot be forged is active, then keys cannot be forged, even if a player is outside the "Forge a key" step of the Timing Chart.
Source: Cannot vs Must