My opponent has 2 Rift Heralds controlling a battlefield. I attack with Irelia and kill both his Heralds. The first Deathknell triggers and he plays Elder Dragon. I Starcross it. Can he play it again with the second Deathknell? Also, if the Irelia dies during the Elder Dragon being played, does she still conquer the battlefield? The Dragon is played during the Deathknell phase of the Rift Herald.

To resolve this situation, we must follow the specific timing and chain mechanics of Riftbound.

Part 1: The Elder Dragon and the Second Deathknell

Yes, your opponent can play a unit with the second Rift Herald Deathknell.

Here is the step-by-step breakdown:

1. Triggering: When both Rift Heralds die, both of their Deathknell abilities trigger. These go on the chain in the order of your opponent's choosing.
2. First Resolution: The first Deathknell resolves. Your opponent plays Elder Dragon (paying its power cost). Elder Dragon enters the battlefield, and its "When you play me" triggered ability is placed on the chain.
3. Reaction: You play Star-Crossed in response to the Elder Dragon's play ability. Star-Crossed resolves, returning both the Elder Dragon and your friendly unit to hand.
4. Second Resolution: The chain then proceeds to the second Rift Herald Deathknell trigger. Because that ability is independent of the first one, your opponent may now play another unit (or a different Elder Dragon if they have one) from their hand as instructed by the second Deathknell.

Part 2: Conquering the Battlefield

If Irelia is killed, she does not conquer the battlefield.

• Rule 348.2.a: A player only establishes control (and thus triggers a "Conquer") if they are the player whose units remain at the battlefield after the showdown closes.
• The Combat Sequence: Combat damage is dealt in the Combat Damage Step, which occurs after the showdown (where the Deathknell triggers happened) has fully closed.
• Result: If Irelia is killed by the Elder Dragon's ability (or combat damage) before the Combat Damage Step finishes, she is moved to the trash. If no friendly units remain at that battlefield at the end of the combat/showdown, you cannot conquer it.

Summary

• Deathknell Interaction: The Deathknell triggers are individual events. Star-crossing the unit played by the first trigger does not prevent the second trigger from resolving and allowing your opponent to play another unit.
• Conquering: To conquer a battlefield, you must have a unit present at the conclusion of combat/showdown. If your units are killed, you lose the opportunity to conquer that battlefield.

This is my interpretation based on Rule 348 and the mechanics of triggered abilities. No official FAQ exists for this specific combination of interactions.