If I have a unit in base, at battlefield 1 and battlefield 2. The enemy plays elder dragon, and I react with smoke and mirrors to swap units at battlefield 1 and base. Who gets killed and why?

To determine who gets killed by Elder Dragon's play effect in your scenario, we must look at how Elder Dragon targets and how your reaction with Smoke and Mirrors changes the battlefield state before the effect resolves.

The Sequence of Events

1. Elder Dragon's Play Effect: When your opponent plays Elder Dragon, its "When I play me..." triggered ability is added to the chain. They must target up to one enemy unit at each location (Battlefield 1, Battlefield 2, and your Base).
2. Reaction: In response, you play Smoke and Mirrors to swap your unit at Battlefield 1 with your unit at your Base.
3. Resolution of Smoke and Mirrors: The spell resolves first (LIFO order). The unit originally at Battlefield 1 moves to your Base, and the unit originally at your Base moves to Battlefield 1.
4. Resolution of Elder Dragon's Ability: The ability checks the validity of its targets.

The Ruling

According to RiftJudge FAQ #9919 and the associated 2026-04-29 Rules FAQ Clarification, Elder Dragon's play effect has strict targeting restrictions tied to specific locations. It targets "up to one enemy unit at [a specific location]."

• When the ability resolves, it checks if the units it originally targeted are still at the locations where they were targeted.
• Because you swapped the units, the unit the opponent targeted at Battlefield 1 is no longer at Battlefield 1, and the unit they targeted at your Base is no longer at your Base.
• These units no longer fulfill the targeting restriction for the Elder Dragon's ability and are unaffected by it.

Result

• The unit at Battlefield 2: Assuming it was targeted and you did not move it, it remains a valid target at that location and will be dealt 1 damage. Since Elder Dragon is on the board, 1 damage is lethal, and this unit will be killed.
• The swapped units: The unit that moved to your Base (formerly at Battlefield 1) and the unit that moved to Battlefield 1 (formerly at your Base) are not dealt damage and will not be killed, as they are no longer at the locations where they were targeted.