When Player A holds a battlefield and plays Charm to move Player B's unit to that battlefield, triggering a showdown, who is the attacker and who is the defender?

Ruling: Player B is the attacker and Player A (who controls the battlefield) is the defender. The attacking player becomes the active player and gets focus first.

Sequence:

• Player A plays Charm while holding the battlefield
• Player B's unit is moved to the battlefield Player A holds
• Showdown is triggered
• Player B becomes the attacker (and active player)
• Player A is the defender (whoever controls the battlefield is always the defender)
• Player B gets focus first as the attacker/active player

Nuances:

• Even though it is Player A's turn when Charm is cast, Player B becomes the active player once the showdown begins because the attacking player always becomes the active player