When does the battlefield `Reaver's Row` effect happen? If I move an attacking unit to the battlefield while my opponent is defending, when can they decide to activate that effect? Say I move a 4 might unit onto a battlefield where the opponent has a 4 might unit. I have a `Falling Star` card in my hand that I want to use, but I don't know if that would be wasted if the opponent can just fall back to base during any time.

To address your questions regarding the timing of Reaver's Row and the use of Falling Star, here is the breakdown based on the official rules and Riftbound mechanics.

1. When does the Reaver's Row effect happen?

Reaver's Row has a "When I defend" trigger. According to Rule 376.4.e, this is a Defend Trigger.

• Trigger Timing: It triggers the moment a unit gains the "Defender" designation during a showdown.
• Chain Placement: It is placed on the "Initial Chain" of the showdown.
• Resolution: You choose a target when the trigger is placed on the chain, but you make the "may" decision (whether to actually move the unit to your base) when the trigger resolves.

2. Can you use Falling Star to prevent the retreat?

Falling Star is an Action spell.

• Timing Restriction: Action spells can only be played during an Open State (when the chain is empty).
• The Showdown Flow: When a showdown begins, an "Initial Chain" is created containing the attack/defend triggers (including Reaver's Row). Because this chain is not empty, you cannot play an Action spell like Falling Star until the Initial Chain has fully resolved.
• The Interaction:
  1. The showdown starts; Reaver's Row goes on the chain.
  2. Players may add Reactions to this chain.
  3. The chain resolves (LIFO). Reaver's Row resolves, and the defender decides whether to move their unit.
  4. Only after the chain is empty and the showdown state becomes "Open" can you play an Action spell like Falling Star.

Conclusion: You cannot use Falling Star to "pre-emptively" kill the unit before the defender makes their choice, because the defender's choice happens during the resolution of the Initial Chain, while you are restricted to playing only Reactions.

3. Does moving the unit to base make Falling Star "wasted"?

No. As confirmed by RiftJudge FAQ #1465, moving a unit to the base does not make it an illegal target for a spell that has already been played.

• If you play Falling Star after the Initial Chain resolves (when the state is Open), and the opponent then reacts with a spell to move their unit to their base, Falling Star will still resolve and deal damage to that unit at the base.
• The unit does not need to be at a battlefield for Falling Star to resolve; it only needs to be a valid target (a unit) when the spell is played.

Summary of the Sequence

1. Showdown Starts: Reaver's Row trigger is placed on the Initial Chain.
2. Initial Chain: Players can only play Reactions. You cannot play Falling Star here.
3. Resolution: The Initial Chain resolves. The defender decides whether to move their unit to their base.
4. Open State: The chain is now empty. You now have priority and can play Falling Star as an Action.
5. Opponent's Response: If the opponent then plays a card to move their unit to their base in response to your Falling Star, the unit will still take the damage when Falling Star resolves, as it remains a legal target.

Note: I could not fully verify this ruling against the retrieved rules.