If Zenith Blade targets a unit and the opponent responds with Retreat to return that unit to hand, does Zenith Blade still resolve or does it fail due to an invalid target?

Ruling: When the targeted unit is returned to hand by Retreat before Zenith Blade resolves, the unit is no longer a legal target. All instructions that reference that unit or use information about that unit will not execute.

Nuances:

• If you use Zenith Blade on an opponent's unit and they Retreat it, you do not get the stun effect since the target is invalid
• If you use Zenith Blade on your own unit and then Retreat it yourself, you still get the stun on the enemy (the stun doesn't require the original target to be valid)