What happens to an active Showdown Chain when you play a unit with Ambush?

When you play a unit with Ambush during an active showdown chain, the following occurs:

1. Timing: Because the Ambush keyword grants the unit [Reaction] timing, you are permitted to play it while the chain is in a "Closed State" (i.e., during an ongoing showdown chain).
2. Chain Interaction: The unit is added to the chain as a pending item.
3. Resolution: Units do not linger on the chain. As soon as it is the unit's turn to be finalized, it resolves immediately, enters the battlefield, and leaves the chain. It does not wait for a round of priority or for other spells on the chain to resolve before entering play.

Critical Considerations

• No Reaction to the Play: While you are playing the unit as a [Reaction], you cannot react to the act of playing the unit itself. Once it is finalized, it enters play immediately.
• Triggered Abilities: While you cannot react to the unit entering the battlefield, you can react to any triggered abilities (such as "When you play me" effects) that occur after the unit has entered play and those triggers have been added to the chain.
• Legality: Per the Unleashed rules update, Ambush is a conditional permissive ability. If the battlefield you are playing the unit to ceases to be a location where you control units before the unit finalizes, the action of playing the unit is undone and the card is returned to your hand.

According to Rule 822.1.b, Ambush grants both the permission to be played to a battlefield where you control units and the [Reaction] timing required to play that unit during a Closed State (such as during a showdown). As clarified in RiftJudge FAQ #9978, units leave the chain directly without a round of priority, unlike spells which linger on the chain.