Can I play Keeper's Verdict to stop a Rengar pouncing from scoring a conquest when my enemy is attacking?

Yes, you can play Keeper's Verdict to remove Rengar, Pouncing from the board, which prevents him from helping your opponent win a combat or conquering the battlefield.

Interaction Details

• Targeting: Keeper's Verdict allows you to choose an enemy unit at a battlefield. Since Rengar is at a battlefield and is an enemy, he is a legal target.
• Resolution: When Keeper's Verdict resolves, the owner of Rengar must place him on the top or bottom of their Main Deck.
• Conquer Prevention: Because a "Conquer" requires units to be present when the Showdown ends, removing Rengar prevents him from contributing to your opponent's unit count. If no other friendly units remain at that battlefield for your opponent, they will not be able to conquer it.

Important Considerations

• Targeting Constraints: You choose your target when you play the spell. Once placed on the chain, the target cannot be changed.
• Timing: Keeper's Verdict does not "counter" the play of Rengar. If Rengar has "When I enter" effects, they will trigger upon his entry to the board before he is removed by your spell.

Note: I could not fully verify this ruling against the retrieved rules.