When can reactions be played in Riftbound, and how do chains and showdowns work with reactions?

Ruling: You can use reactions during your turn or during your opponent's turn only when you gain priority - either when there's an active chain and your opponent passes priority to you, OR during a showdown when you gain focus (and therefore priority). A chain must be started first; base speed discretionary actions (like playing vanilla units or moving to a battlefield you control) do not open a chain where you can react.

Sequence:

• A chain is started when spells or actions are cast
• When a unit moves to a battlefield that is not controlled, an open showdown is started
• During a showdown, you can play reactions when you have focus/priority
• Your opponent can respond to your attempt to take a battlefield with an action or reaction

Nuances:

• You cannot use reactions during your main phase just because you have priority - there must be an active chain
• Moving to an unoccupied battlefield triggers an open showdown
• Hidden cards are not played when hidden; they can be reacted with on the next turn