If my opponent moves to an unoccupied battlefield, and I ambush there with Rengar, Trophy Hunter, who is attacking and who is defending?

To determine the attacker and defender designations in this scenario, we must look at the rules for Contested status and the specific play conditions of Rengar, Trophy Hunter.

The Ruling

• Attacker: Your opponent (the player who first moved their unit to the open battlefield).
• Defender: You (the player who plays Rengar, Trophy Hunter to that same battlefield).

Reasoning

1. Attacker Designation: According to Rule 442.1.a.1, the player who first causes a battlefield to become Contested is designated as the Attacker. When your opponent moved their unit to an unoccupied battlefield, they initiated a non-combat showdown and applied the Contested status, making them the Attacker.
2. Defender Designation: By playing Rengar, Trophy Hunter to that battlefield while it is already contested by your opponent, you become the Defender (Rule 442.1.a.2).
3. Playing Rengar, Trophy Hunter: It is important to clarify that Rengar, Trophy Hunter's ability to be played to a battlefield without your units is not an effect of the Ambush keyword. Per Rule 822.1.b, the Ambush keyword generally allows a unit to be played to a battlefield where you already control units. Rengar, Trophy Hunter possesses a unique, specific ability—distinct from Ambush—that explicitly grants permission to be played to a battlefield where there are enemy units, even if you control no units there. Because the battlefield is Contested and an enemy unit is present, you meet the requirement to play him as a [Reaction] during the ongoing showdown.

Summary of Sequence

1. Opponent's Move: Your opponent moves a unit to an open battlefield. The battlefield becomes Contested, and your opponent is designated as the Attacker.
2. Your Response: During this active non-combat showdown, you play Rengar, Trophy Hunter as a [Reaction]. Because he is being played to a battlefield with enemy units, this is a legal move.
3. Designation: Upon playing Rengar, Trophy Hunter to the contested battlefield, you are designated as the Defender.
4. Showdown Continues: The showdown proceeds. Once the chain empties and both players pass, the non-combat showdown transitions into a combat showdown, at which point you are officially the Defender.

This interpretation is based on the rules for Contested status (Rule 445/446), the definitions of Attacker/Defender designations (Rule 442.1.a), and the specific card text of Rengar, Trophy Hunter.