If an opponent tries to conquer a battlefield with a unit, and then I react with [Rengar the trophy hunter], who is the attacker? Or how does it resolve in detail? Is there a difference if I react in showdown or on attack trigger?

When an opponent moves a unit to an empty battlefield, they initiate a non-combat showdown and are designated the Attacker. If you play Rengar, Trophy Hunter in response to this move, you become the Defender.

Here is the detailed resolution of the sequence and the breakdown of how these designations work.

1. The Sequence of Events (Detailed)

1. Opponent's Move: Your opponent moves a unit to an unoccupied battlefield. The battlefield becomes Contested. According to Rule 442.1.a.1, this player is designated as the Attacker.
2. Your Response: You play Rengar, Trophy Hunter as a [Reaction]. Because there is an enemy unit present at the contested battlefield, Rengar's special ability allows you to play him there.
3. Designation: Per Rule 442.1.a.2, because you played a unit to a battlefield that is already contested by your opponent, you are designated as the Defender.
4. Showdown Continues: The showdown remains ongoing. Because units from both players are now present at the contested battlefield, a combat will begin once the chain empties and both players pass focus.
5. Combat Designation: When the combat officially begins, your opponent remains the Attacker and you remain the Defender.

2. Is there a difference if you react in a showdown vs. on an attack trigger?

Yes, there is a significant difference in timing and state:

• Reaction to Move (Showdown): This occurs during the initial non-combat showdown. The battlefield is Contested, but combat has not officially begun. You are playing Rengar to a battlefield that is in a state of contention. This leads to the standard "surprise defense" scenario where combat will trigger once the current chain resolves and focus is passed.
• Reaction to Attack Trigger: An Attack Trigger (e.g., "When I attack...") only occurs after a combat has officially opened and a unit has already gained the Attacker designation (Rule 459.2.b.1). If you wait to play Rengar until after an attack trigger has been placed on the chain, you are responding to an already active combat. The designations are already locked in (Opponent is Attacker, you are Defender), and you are simply adding Rengar to the existing combat.

Summary of Rules

• Attacker Designation: Determined by the player who first causes a battlefield to become Contested (Rule 442.1.a.1).
• Defender Designation: Determined by the player who enters the contested battlefield after the Attacker has already established the contested status (Rule 442.1.a.2).
• Rengar's Permission: Rengar, Trophy Hunter does not have an "Ambush" keyword ability; he possesses a specific unique permission that allows him to be played as a [Reaction] to any battlefield with enemy units, effectively bypassing the standard restriction that you must already control units at that location to use Ambush (RiftJudge FAQ #9992).

This interpretation is based on Rule 442.1.a regarding Attacker/Defender designations and the established RiftJudge FAQ rulings #9820, #9992, and #10213 for Rengar, Trophy Hunter.