From Archon Arcana - The KeyForge Wiki
Key Hammer • Dis • Action • Uncommon • Play: If your opponent forged a key on their previous turn, unforge it. Your opponent gains 6. • Artist: Caravan Studio • Card Number: CotA:066
Key Hammer only affects a single key. You will choose one of the two keys your opponent forged on their previous turn and unforge it.Source: Rulebook
My opponent forges two keys on their turn. On my next turn, I play Key Hammer (CoTA 66). What happens?
Key Hammer only affects a single key. You will choose one of the two keys your opponent forged on their previous turn and unforge it.
In a previous turn, I played a Key Hammer to unforge my opponent's first key, leaving them with zero keys currently forged. On my current turn, I play Furtive Investors, do I get to gain an Æmber for the key they had forged but I unforged?