Can a player respond to Reaver's Row's battleground effect by revealing a hidden card in the face-down zone, and can that same hidden unit be the one that moves back to base?

Ruling: You can respond to Reaver's Row's trigger by revealing hidden cards at Reaction speed. However, you cannot target a face-down hidden unit with Reaver's Row's effect when it goes on the chain, so the hidden unit cannot be the one moving back to base.

Sequence:

• Reaver's Row triggers at the start of combat and goes on the chain
• You must target a valid (face-up) unit when the trigger goes on the chain
• You can then respond by revealing a hidden card at Reaction speed
• The hidden card resolves first
• Then Reaver's Row resolves, moving the previously targeted unit back to base

Nuances:

• If another trigger happens before Reaver's Row (like Noxian Drummer attacking or Merchant moving), you can reveal the hidden card in response to that earlier trigger, making it a valid face-up unit before Reaver's Row triggers
• Once revealed through this earlier trigger, the unit can then be targeted by Reaver's Row when it goes on the chain