Opponent moves his unit on an open Battlefield, attempting to conquer during the showdown. I cast Rengar, Trophy Hunter on this Battlefield. Who is the attacker and defender?

The Ruling

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

Reasoning

1. Attacker Designation: According to Rule 442.1.a.1 (and confirmed by FAQ #8839, #1485, and #9032), the player who first causes a battlefield to become Contested is designated as the Attacker. When your opponent moved their unit to the empty battlefield, they 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: Regarding the legality of playing Rengar, Trophy Hunter in this situation: Because your opponent has already moved their unit to the battlefield, there is an enemy unit present. This satisfies the condition for playing the card at that location as a [Reaction] during the ongoing showdown, even though you do not currently have other units there.

Summary of Sequence

1. Opponent's Move: Your opponent moves a unit to an open battlefield. It becomes Contested and a showdown begins. Your opponent is the Attacker.
2. Your Response: During the showdown, you play Rengar, Trophy Hunter. Because an enemy unit is present, you may play him to that battlefield as a [Reaction].
3. Showdown Continues: The showdown proceeds. Because units from both players are now present at the contested battlefield, a combat will begin once the current showdown chain empties and the game returns to a neutral open state.
4. Combat Designation: When the combat begins, your opponent is officially designated as the Attacker and you are designated as 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.

Note: I could not fully verify the specific card text for Rengar, Trophy Hunter against the retrieved rules.