I have a unit and a hidden Facebreaker at a battlefield, and my opponent moves a unit into the battlefield. If I reveal Facebreaker, what happens to our units?
To determine what happens, we must look at the requirements for playing Facebreaker and the rules regarding combat and showdowns.
Ruling
If you reveal and play Facebreaker from your hidden zone, you must target one friendly unit and one enemy unit at that battlefield. Once resolved, both units will be stunned (they will not deal combat damage this turn).
Because your opponent moved a unit into your battlefield, this likely triggered a Showdown (Rule 429.1). Here is how the interaction plays out:
- Targeting: According to the FAQ for Facebreaker, you must have both a friendly unit and an enemy unit present at the battlefield to play the card. Since your opponent moved a unit in, both targets are available.
- Effect: Both units are stunned. Stunned units do not deal combat damage during the showdown.
- Combat Outcome: If the showdown ends and units from both players are still present at the battlefield, the attacker's units are recalled to their base (Rule 444.1.a.2).
Important Nuances
- Stunned Units: Even if a unit is stunned, it is still considered "present" at the battlefield. If the showdown concludes with both your unit and your opponent's unit still there, the attacker's units will be recalled.
- Timing: You must play Facebreaker while the showdown is open. If you wait until after the showdown has fully resolved (and no other triggers are pending), you will no longer be able to play it.
- Recall: If the combat ends and both players still have units at the battlefield, the attacker's units are recalled to their base. This is a standard result of a combat showdown that does not end in a clear conquer (see the FAQ regarding "Showdown between two units").
Summary: Playing Facebreaker will stun both units, preventing them from dealing damage. If both units remain on the battlefield after the showdown, the attacker's units will be recalled to their base.