Can an Action Spell be played during a showdown on defense after the attacker's spell has resolved?

Ruling: Yes, after an attacker's spell has resolved during a showdown, you can play an Action Spell. You can only respond to an action with a Reaction before it resolves, but once their action resolves, you return to Showdown Open state and can play your own Action.

Sequence:

• Attacker plays an Action Spell (creates a chain, entering Showdown Closed state)
• Defender can only respond with Reactions while the chain exists
• Once the attacker's spell resolves and the chain completes, the showdown returns to Open state
• Defender can now play their own Action Spell, starting a new chain

Nuances:

• Action Spells can be played during showdowns in both offense and defense positions
• The key distinction is between Showdown Open state (no chain exists, Actions allowed) and Showdown Closed state (chain exists, only Reactions allowed)
• When focus is passed to you during a showdown, you can play an Action