The opponent has units on both battlefields. They play a card with Flash, and in response, I want to play Rengar, Trophy Hunter onto the battlefield. Do we resolve the combat first, or do the units retreat and I receive a point for the capture?

To determine the outcome of this situation, we must follow the sequence of the Chain and the rules for combat and state changes.

Sequence of Events

1. Opponent's Play: The opponent plays a card with "Flash" (a spell or ability that moves units). This is placed on the Chain as a pending item. The state of the turn is now Closed.
2. Your Response: You play Rengar, Trophy Hunter as a [Reaction]. Because there is an enemy unit at the battlefield, his ability allows you to play him there. This unit spell is placed on top of the Chain (LIFO).
3. Chain Resolution:
   • The Chain resolves in reverse order (Last-In, First-Out).
   • Rengar, Trophy Hunter resolves first, entering the battlefield.
   • Then, the opponent's "Flash" card resolves. If that effect moves their units to a different location (such as back to their base), they are no longer present at the battlefield.

Does Combat Happen?

Whether combat occurs depends on the state of the battlefield after the Chain finishes resolving:

• If your units and your opponent's units are both present at the battlefield after the Chain resolves: The showdown transitions into a combat showdown. You must resolve combat (Combat Damage Step followed by Combat Cleanup) before anyone can Conquer or score the battlefield.
• If your opponent's units have moved away due to the "Flash" effect and only your units remain: The combat/showdown is cancelled because there are no longer opposing units present to contest the battlefield. You would then be the only player with units at that location.

Will you receive a point?

You do not immediately receive a point. Points for "Conquering" are awarded during the Resolution step if you establish control of a battlefield where you have units remaining and the opponent does not (Rule 461.5.d).

Important Note: If you are the only player with units at the battlefield after the chain resolves, you will Establish Control during the cleanup process. If you have not yet scored that specific battlefield this turn, you will then Conquer it and gain the point.

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 Flash and Rengar.