If Bloodharbor Ripper returns his only unit at BF with the tap ability, and I react with Rengar Trophy Hunter to it, my Rengar gets played, then his unit gets returned, and then the open showdown starts? Or is it combat showdown, or does the showdown start with still his unit on the board?

To determine the exact outcome, we must follow the Chain (Last-In, First-Out) and the rules regarding Showdown types and combat staging.

Sequence of Events

1. Ability Activation: Your opponent activates Bloodharbor Ripper's ability, targeting their only unit at that battlefield. The ability is placed on the Chain. The battlefield is currently controlled by your opponent.
2. Reaction: You play Rengar, Trophy Hunter as a [Reaction] to the activated ability. Because an enemy unit is present, Rengar's special permission allows you to play him to that battlefield.
   • Rengar is now added to the top of the Chain.
3. Chain Resolution (LIFO):
   • First: Rengar, Trophy Hunter resolves and enters the battlefield.
   • Second: Bloodharbor Ripper's ability resolves. It returns the opponent's unit to their hand.
4. State After Resolution: The opponent's unit is gone, and Rengar remains on the battlefield.
5. Showdown Determination:
   • Because the battlefield was Contested (you played a unit to a location you don't control), a showdown is staged.
   • However, since your opponent’s unit was returned to their hand by the Ripper's ability, only your unit remains at the battlefield.
   • A Combat Showdown is only staged if units controlled by different players are present at the battlefield (Rule 323.9, Rule 456). Since the opponent has no units there, no combat will occur.

Final Result

• Showdown: A Non-Combat Showdown begins.
• Conquer: Because only your unit remains at the battlefield, you will establish control (Rule 348.2.a) and successfully conquer the battlefield, provided you have not already scored it this turn.

Summary: Your Rengar arrives, the opponent's unit departs, and you secure the battlefield without combat. You do not have to fight because the opponent's unit is no longer there to contest control.

This is my interpretation based on the mechanics of the Chain (LIFO resolution) and the rules for Establishing Control and Conquering (Rule 461.5). No official FAQ exists for this exact combination of Bloodharbor Ripper and Rengar.