If I Star Crossed an elder dragon on its play trigger and they react with Flurry of Blades - do all my units at battlefields die?
Ruling
Yes, your units at the battlefields will die.
Even if you successfully return the Elder Dragon to its owner's hand using Star-Crossed, the Elder Dragon player's passive ability—"Any amount of your damage is enough to kill enemy units"—remains in effect for damage dealt by that player while the effect is active, and the 1 damage dealt by Flurry of Blades will be lethal to your units.
The Sequence
- Opponent plays Elder Dragon: Its "When you play me" ability is placed on the chain. Your opponent locks in targets for its damage.
- You play Star-Crossed (Reaction): You target the Elder Dragon (enemy unit) and one of your units (friendly unit), returning both to their owners' hands.
- Opponent plays Flurry of Blades (Reaction): This is placed on top of the chain.
- Chain Resolution (LIFO):
- Flurry of Blades resolves: It deals 1 damage to all units at all battlefields.
- Lethality check: Because the Elder Dragon player's passive ability is active, the 1 damage from Flurry of Blades is considered lethal to all your units at the battlefields. Your units are marked with lethal damage and sent to the trash.
- Star-Crossed resolves: The Elder Dragon is returned to its owner's hand. If your chosen friendly unit was already killed by Flurry of Blades and sent to the trash, it cannot be returned to your hand.
- Elder Dragon's play trigger resolves: The ability attempts to resolve using its original targets.
Why your units die
- Elder Dragon's Passive: According to RiftJudge FAQ #8684, the passive ability "Any amount of your damage is enough to kill enemy units" means that any damage dealt by the Elder Dragon player is sufficient to kill units they do not control.
- Persistence of Lethality: Even though Star-Crossed removes the Elder Dragon from the board, it does not retroactively heal your units or remove the lethality of the damage they already took. As confirmed by RiftJudge FAQ #9875, the passive ability modifies the rules of combat/damage for the controller; it does not require the Elder Dragon to remain on the battlefield for the damage to remain lethal.
- Flurry of Blades: Because the Elder Dragon player is the controller of Flurry of Blades, the 1 damage it deals to your units is "their damage," which, per the passive, is enough to kill your units.
Note: I could not fully verify this ruling against the retrieved rules.