If I move a merchant into an empty battlefield and pass priority, can I respond to my own move trigger with Rengar, and if so, does Rengar land at base or the same battlefield?

Ruling: Rengar can only be played to base in this situation because you are not attacking the previously uncontrolled battlefield.

Nuances:

• Moving into an empty/uncontrolled battlefield does not count as attacking it
• Rengar's ability to enter a battlefield requires you to be attacking that battlefield