Can Falling Comet be played in an open show down, when an opponent moves a unit to an uncontested battlefield?

Yes, Falling Comet can be played in a showdown that occurs when an opponent moves a unit to an uncontested battlefield.

Explanation and Timing

According to RiftJudge FAQ #9709 and Rule 344, when a unit moves to an uncontested battlefield, a Showdown begins. During a Showdown, the game state allows for both [Action] and [Reaction] cards to be played.

• Falling Comet has the [Action] keyword.
• Since the movement of the unit has triggered a Showdown, the game enters a Showdown Open state, allowing you to play Falling Comet to start a chain.

Sequence of Events

1. Showdown Begins: The opponent moves their unit to an uncontested battlefield, triggering a Showdown (Rule 446.1).
2. Priority/Focus: The attacking player (the player who initiated the move) gains focus first.
3. Playing the Card: If the opponent passes focus, you may play Falling Comet (as an [Action]).
4. Chain Created: Playing Falling Comet creates a chain (Rule 338.1). The game state becomes Showdown Closed.
5. Resolution: Players may then add [Reaction] cards to the chain (alternating turns). Once both players pass, the chain resolves in reverse order (LIFO).

Important Interaction: Falling Comet Damage

As per RiftJudge FAQ #5876:

• If Falling Comet deals damage equal to or greater than the target unit's might, the unit is destroyed before the combat damage step of the showdown.
• If the unit is destroyed by the spell, it does not deal combat damage.

Note: I could not fully verify this ruling against the retrieved rules, specifically regarding the specific damage value of the card or the exact card text permissions.