From Archon Arcana - The KeyForge Wiki

If the active player has enough Æmber to forge a key during this step, they must do so. To forge a key, the active player spends Æmber from their Æmber pool equal to their current key cost, then they flip any one of their key tokens to its forged side.

The default key cost is six Æmber (6Aember icon). Some card abilities may increase or decrease this cost. Key cost cannot be less than zero.

No more than one key can be forged during this step each turn, even if the active player has enough Æmber to forge multiple keys. Some cards have effects that allow Æmber on those cards to be spent when forging keys. If the active player controls such a card, the Æmber on it can be spent along with Æmber in the active player‘s Æmber pool, in any combination.

Spent Æmber is returned to the common supply.

A player immediately wins the game when they forge their third key.

KeyForge Master Rulebook v17.3 March 2024