When is the last moment you could play a hidden card during a losing showdown if the opponent doesn't trigger anything?

Ruling: During a losing showdown, the last moment to play a hidden card is at action speed timing when the defender has priority, before both players consecutively pass priority without doing anything (which would complete the showdown).

Sequence:

• Showdown is initiated
• Players alternate getting priority at action speed timing
• The defender (player with the hidden card) gets priority
• This is the last moment to play the hidden card
• If both players consecutively pass priority without doing anything, the showdown completes and damage is assigned

Nuances:

• Hidden cards gain the ability to be played at reaction speed
• Hidden cards with location-relevant effects gain the additional text "here" (only affecting the battlefield where they are hidden)
• The hidden card must have legal targets to be played, otherwise it moves to the graveyard when the battlefield changes state
• If there's already a chain/spell being cast, that's never the "last moment" - you can respond to that chain before it resolves