Does Flurry of Blades activate Elder Dragon Passive which kills enemies when they take damage?
Yes, playing Flurry of Blades while you control Elder Dragon will cause all enemy units at battlefields to be killed.
According to RiftJudge FAQ #8684, this interaction works as follows:
- The Interaction: Elder Dragon’s passive ability states: "Any amount of your damage is enough to kill enemy units." When you cast Flurry of Blades, you are the controller responsible for the spell, and thus responsible for the "deal" action.
- The Sequence:
- You play Flurry of Blades.
- The spell resolves, dealing 1 damage to all units at all battlefields.
- Because you control Elder Dragon, your damage is subject to its passive ability. Any enemy unit that takes that 1 damage is immediately considered to have received lethal damage and is killed.
- Your own units at the battlefields also take 1 damage from the spell, but because the passive only applies to "enemy units," they survive (provided the 1 damage is not lethal to them normally).
As confirmed in the FAQ, Elder Dragon's ability does not require the damage source to be the Elder Dragon itself; it only requires that the damage be yours and that the target be an enemy unit. Since Flurry of Blades is a spell you control, the damage it deals to enemy units is "your damage," triggering the lethal threshold modification.