Can I play Rengar during the chain created by Travel Merchant in Battle Phase, and if so, does he join as an attacker?

Ruling: Yes, Rengar can be played during the chain created by Travel Merchant, but he goes to base and cannot join as an attacker on that move.

Sequence:

• Travel Merchant starts a chain during Battle Phase
• Rengar can be played as a reaction during this chain
• Rengar enters play to base
• Rengar cannot join as a potential attacker for that specific move

Nuances:

• Even though it's during Battle Phase, the combat hasn't started yet so there are no attackers/defenders at the point when the chain is resolving
• Rengar can be played anytime a reaction can be done, but joining as an attacker requires the combat to have already begun