If you play Undertitan and then attack where you play Rengar as a reaction, does Rengar get the +2 from Undertitan?

Ruling: If you play Undertitan normally and then play Rengar as a reaction, Rengar does not get the +2 bonus because Undertitan's ability only affects units already on the board when it resolves. However, if you reveal and play Undertitan off Reksai/Swarm Queen's attack ability, you can sequence it so Rengar gets the bonus.

Sequence: (when using Reksai/Swarm Queen to reveal Undertitan)

• Attack with Reksai/Swarm Queen, triggering its reveal ability
• Reveal and play Undertitan from the reveal
• Undertitan's "when you play me, give +2" ability goes on the chain
• Play Rengar as a reaction to Undertitan's ability (before it resolves)
• Rengar resolves and enters the battlefield
• Undertitan's ability resolves, giving +2 to all units you control (including Rengar)

Nuances:

• The key difference is whether Undertitan's ability has already resolved or not when Rengar enters play
• You can chain Rengar to either the Swarm Queen attack or to Undertitan's on-play ability
• With multiple Reksai/Swarm Queens, you can stack multiple Undertitan triggers, but each resolves sequentially