I have Elder Dragon and Anivia in base. I move my Anivia to a base where an enemy has units. The enemy reacts with Rengar Trophy Hunter. Does Rengar take damage since he was played as a reaction before Anivia's ability resolved?

To determine if Rengar, Trophy Hunter takes damage, we must look at the timing of the chain and the resolution of Anivia's ability.

Ruling

Yes, Rengar, Trophy Hunter will take 3 damage from Anivia's ability.

Reasoning

1. Anivia's Trigger: When you move Anivia to the battlefield, her "When I attack, deal 3 to all enemy units here" ability triggers and is placed on the chain.
2. Reaction: The enemy plays Rengar, Trophy Hunter as a reaction to Anivia's triggered ability. Rengar enters the battlefield.
3. Chain Resolution: The chain resolves in reverse order (LIFO). Rengar enters play first, then Anivia's triggered ability resolves.
4. Damage Execution: When Anivia's ability resolves, it checks the battlefield for enemy units. Since Rengar is now present at that battlefield, he is included in the "all enemy units here" instruction and is dealt 3 damage.
5. Elder Dragon's Interaction: Elder Dragon's ability states: "Any amount of your damage is enough to kill enemy units." Because Anivia's ability is a triggered ability of a unit you control, the damage she deals is considered "your damage." Therefore, if Rengar's current Might is 3 or less, he would be killed instantly by this damage. If his Might is greater than 3, he survives with 3 damage marked on him.

Summary of Sequence

• Anivia moves to the battlefield, triggering her ability.
• Anivia's ability is placed on the chain.
• The opponent plays Rengar, Trophy Hunter as a reaction to the ability.
• Rengar enters the battlefield.
• The chain resolves: Anivia's ability deals 3 damage to all enemy units at that location, including Rengar.
• Because this is "your damage," Elder Dragon's ability applies, and Rengar is killed if the 3 damage is equal to or greater than his current Might.