Keyfrog
From Archon Arcana - The KeyForge Wiki
Keyfrog • Untamed • Creature • Common • Destroyed: Forge a key at current cost. • Artist: Marko Fiedler • Card Number: MM:369, DT:419, VM2023:371, VM2024:375, MoMu:422, ToC:308
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Official Rulings
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Yes, your opponent will forge a key through the resolution of Keyfrog’s “Destroyed:” ability. Forging is not optional; if a player has enough Æmber to forge, including Æmber on cards like Senator Shrix, they must do so. The active player will make the decision regarding how much Æmber will be taken off of Senator Shrix and how much will come from the opponent's pool.Source: Rulebook
My opponent has 5 
in their pool, a Senator Shrix (WC 193) with 5 on it, and a Keyfrog (MM 369). Their key cost is 6. If I destroy their Keyfrog, do they forge a key? If so, where does the æmber come from?
in their pool, a Senator Shrix (WC 193) with 5 on it, and a Keyfrog (MM 369). Their key cost is 6. If I destroy their Keyfrog, do they forge a key? If so, where does the æmber come from?Yes, your opponent will forge a key through the resolution of Keyfrog’s “Destroyed:” ability. Forging is not optional; if a player has enough Æmber to forge, including Æmber on cards like Senator Shrix, they must do so. The active player will make the decision regarding how much Æmber will be taken off of Senator Shrix and how much will come from the opponent's pool.
Source: Rulebook
Commentary
Keyfrog's Destroyed: ability resolves before Keyfrog leaves play. For Keyfrog's ability to resolve, Keyfrog's controller must have enough Æmber in their pool at the time the ability resolves, which is before events like captured Æmber would return to a player's pool and also before abilities that resolve based on Keyfrog's destruction (e.g. "after a creature is destroyed" abilities or "if this damage destroys a creature" abilities) would resolve.