If I react to Elder Drake with flash and bring back the units he targeted, will it still kill them or will it mistargeted?

The units targeted by Elder Dragon's "When you play me" triggered ability will not be dealt damage, and they will not be killed.

This is because Elder Dragon’s triggered ability contains specific targeting restrictions—it requires targeting enemy units at specific board locations (e.g., "up to one enemy unit at each location").

According to RiftJudge FAQ #9919 and the associated Rules FAQ Clarification (2026-04-29):

"If you play Flash in reaction to the play effect of Elder Dragon, any units that are moved to a location other than the one where they were targeted will no longer fulfill the targeting restriction of the Elder Dragon play effect and will be unaffected by the ability as it resolves."

Sequence of Events

1. Play Phase: You play Elder Dragon. Its triggered ability is placed on the chain, targeting specific units at specified locations (e.g., Battlefield A, Battlefield B).
2. Reaction Window: You play Flash in response.
3. Resolution (LIFO):
   • Flash resolves first: Your units are moved to your base.
   • Elder Dragon's triggered ability resolves: It checks the location of the targeted units. Because the units are no longer at the battlefields where they were targeted, they no longer fulfill the targeting restriction. Consequently, they are unaffected by the ability.