If my opponent has 1 unit on a BF when I attack and he then ambushes a unit in, what happens if I react to the ambush with a Star-Crossed to bounce back the only unit he had pre-Ambush?

To determine what happens when you use Star-Crossed in this scenario, we must look at the timing of your reaction and the targeting requirements of the spell.

The Ruling

Yes, you can play Star-Crossed, and if it resolves, it will bounce your chosen friendly unit and the pre-Ambush unit back to their owners' hands. However, your opponent's new Ambush unit will remain on the battlefield, and because it is now the only unit present for your opponent at that location, they will retain control of the battlefield.

Sequence of Events

1. Opponent's Action: You attack, and your opponent plays an Ambush unit to the contested battlefield.
   • Note: Per RiftJudge FAQ #9978, you cannot react to the playing of the Ambush unit itself. It finalizes and enters the battlefield immediately.
2. Your Reaction: You play Star-Crossed as a reaction.
   • Requirement: Per RiftJudge FAQ #9452 and FAQ #9865, Star-Crossed requires both a valid friendly unit and a valid enemy unit to be targeted to be played. As long as you have a friendly unit on the board and your opponent has their pre-Ambush unit on the board, Star-Crossed is a legal play.
3. Chain State: The chain now contains:
   • [Top] Star-Crossed
   • [Bottom] (Any triggered abilities from the Ambush unit entering play, if applicable)
4. Resolution (LIFO):
   • Star-Crossed resolves first. It returns your targeted friendly unit and the opponent's pre-Ambush unit to their owners' hands.
   • The opponent's Ambush unit remains on the battlefield.

Key Considerations

• Battlefield Control: According to Rule 187.4.c (from the Unleashed update), control of a battlefield is not lost while there are items on the chain. Once the chain resolves and Star-Crossed finishes, your opponent still has one unit (the Ambush unit) at the battlefield. Therefore, they do not lose control of the battlefield, and they maintain their status as the Defender.
• Valid Targets: Because Star-Crossed targets the pre-Ambush unit, it is a legal target even though it is at a battlefield where a showdown is ongoing. Once it is returned to the owner's hand, it is no longer on the board, but the Ambush unit remains to represent the opponent's presence.

This interaction is based on the targeting requirements for spells like Star-Crossed and the rules regarding battlefield control and showdowns. No official FAQ specifically covers this exact "bounce the first unit" scenario, but it follows directly from the established rules for targeting and chain resolution.