Can Last Breath be used on a unit that is already mid-combat in a showdown, specifically to respond to a Reaction spell like Hidden Blade?

Ruling: No, Last Breath cannot be used to respond to Reaction spells during a showdown because it is an Action spell, not a Reaction. Actions can only be played while the Chain is empty.

Sequence:

• Start of Showdown - opponent passes
• You play Hidden Blade (Reaction) onto opponent's unit and pass
• Opponent can only play a Reaction spell or pass at this point
• If opponent passes, Hidden Blade resolves and the unit dies
• Only after the Chain is empty can the opponent play Last Breath on a different unit

Nuances:

• Last Breath can be played on any unit, not just Yasuo (signature spells are only restricted during deck construction, not targeting)
• Actions can never react to anything at any time - only Reactions can react