During a showdown, if I pass priority and my opponent plays a reaction spell (Discipline), can I play an action spell (Rune Prison) after their reaction resolves?

Ruling: Yes, you can play an action spell after the reaction resolves. When the chain empties (after Discipline resolves), you regain focus and priority, allowing you to play an action spell on the empty chain.

Sequence:

• Opponent plays Discipline as a reaction
• Discipline resolves and the chain empties
• Focus and priority return to you
• You can now play Rune Prison as an action on the empty chain

Nuances:

• Action spells can only be played on an empty chain
• You cannot chain an action spell to a reaction spell that's already on the chain
• Focus passes when the chain empties completely
• Showdowns are not limited to 3 chains