If I have focus on a showdown and start with an action, can I instantly play a reaction or does priority pass automatically to my opponent?

Ruling: When you add a chain item (action or reaction), priority first goes to the player who added it. So yes, you can play a reaction immediately after your action before passing priority to your opponent.

Sequence:

• Player with focus plays an action
• That player retains priority and can add reactions
• Player can continue adding reactions until they pass priority
• Once priority is passed, opponent gets priority to respond
• Priority passes back and forth as players add chain items
• When both players pass consecutively, the top chain item resolves
• After resolution, priority goes to whoever controls the next item on the chain

Nuances:

• You cannot chain block in Riftbound - even if you play a reaction after your action, your opponent still has full opportunity to respond to the original action
• Adding a chain item gives you priority back, but you must pass it for your opponent to respond
• The defender will always be able to respond to any chain item, though it may be delayed if the active player adds multiple items before passing
• You could theoretically add many reactions before passing priority, but this can be risky (e.g., if opponent counters one, you may waste the others)