Is there timing before a Showdown starts where reactions can be played, and do abilities like Assault, Shield, or 'When I attack/defend' triggers open chains that can be reacted to?

Ruling: You cannot react to a unit moving into the battlefield before the showdown starts. Assault and Shield are passive abilities that don't go onto the chain, but 'When I attack/defend' triggers do go onto an initial chain once the showdown has already begun.

Sequence:

• Unit moves to battlefield with another unit present
• If the moving unit has 'when you move me' triggers, those must be fully resolved before the showdown begins
• Showdown begins
• 'When I attack/defend' triggers go onto the initial chain (already within combat steps)

Nuances:

• The only exception for pre-showdown timing is 'when you move me' triggers (like Travelling Merchant), which must be fully resolved before the staged combat/showdown begins
• If moving to the battlefield triggers no abilities or printed text, there are no pre-combat chain opportunities to react