When an opponent attempts to conquer an uncontrolled battlefield and you Zenith Blade your own unit there during the showdown: (1) Can you play Shen on that battlefield during the showdown? (2) If your opponent stuns your unit during the showdown, who retreats when it resolves?

Ruling: You cannot play Shen on the battlefield during the showdown because no one controls it yet. If your opponent stuns your unit during the showdown, only the opponent's unit retreats (as the attacker), while your stunned unit can still take the spot and establish control at the end of the showdown.

Sequence:

• Opponent initiates showdown to conquer uncontrolled battlefield (becoming the attacker)
• You play Zenith Blade to move your unit to the battlefield
• During showdown, you cannot play Shen because you don't control the battlefield yet
• If opponent stuns your unit, showdown resolves with opponent's unit retreating as the attacker
• Your stunned unit remains and establishes control over the battlefield

Nuances:

• A stunned unit can still take a battlefield spot and establish control at the end of a showdown
• The opponent is designated as the attacker because they contested the uncontrolled battlefield on their turn, which is why their unit retreats when combat ends