If I am attacking an enemy contested battlefield with one unit by playing "Rengar, Trophy Hunter" while a trigger for "The Syren" is on the stack, what happens?

To resolve this interaction, we must look at how the Chain functions during a Showdown.

The Ruling

When you play Rengar, Trophy Hunter in response to an existing trigger on the Chain, his entry into the game and any resulting "When you play me" triggered abilities are added to the top of the Chain. Per the LIFO (Last-In, First-Out) resolution principle, the items added most recently to the Chain resolve before the older items.

Sequence of Events

1. Trigger on Chain: The Syren's trigger is already on the Chain. The game is currently in a Closed State (Rule 331.1).
2. Rengar Played: You play Rengar, Trophy Hunter as a [Reaction]. Because this is a unit card being played, it is added to the Chain as a spell. It will not enter the battlefield until its resolution.
3. Rengar's Triggered Ability: If Rengar, Trophy Hunter had a "When you play me" triggered ability, it would be added to the top of the Chain as a new Pending Item.
4. Chain Resolution (LIFO):
   • First: The newest item on the Chain (Rengar’s "When you play me" trigger, if applicable) resolves.
   • Second: The Rengar, Trophy Hunter unit spell resolves, placing him onto the battlefield.
   • Third: The Syren's trigger resolves.

Clarifications

• Role Designation: Playing Rengar, Trophy Hunter onto a contested battlefield designates you as the Defender.
• State of the Turn: Because a Chain already exists, the turn is in a Closed State (Rule 331.1). In this state, you may only play cards or abilities with the [Reaction] keyword (Rule 309.1.a).
• LIFO Resolution: According to the LIFO (Last-In, First-Out) timing rule, the Chain resolves in the reverse order of items added. Rule 340.1 states: "The newest item on the Chain resolves." Therefore, the Rengar unit spell (and any associated triggers) will resolve before the The Syren trigger that was already waiting on the Chain.

This interpretation is based on Rule 309 (Open/Closed States), Rule 331 (Closed State definition), Rule 340 (Chain Resolution/LIFO), and the timing rule for LIFO resolution.

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