When an opponent moves a unit to an uncontrolled/empty battlefield, does that open a chain or start a showdown? Does the defending player get focus and can they play actions?

Ruling: Moving to an empty battlefield starts a showdown. The attacking player gets first chance to play an action or reaction to start a chain, and if they pass, focus passes to the defender who can then play an action or reaction.

Sequence:

• Opponent moves to empty battlefield, showdown opens
• Attacking player gets first chance to play action or reaction to start a chain
• If they pass, focus passes to defender who can play action or reaction
• Focus goes back and forth until both players pass consecutively
• If attacker plays an action or reaction, defender can only react
• Once that chain resolves, focus passes to defender regardless of who was last on the chain

Nuances:

• The defender doesn't "pass" on a reaction to get to their action - if the attacker passes initially, the defender can play either an action or reaction
• In showdown state, after a chain resolves, focus passes to the other player regardless of who finalized the last thing on the chain