When attacking a contested battlefield, if I pass without playing an Action spell and my opponent plays an Action spell, can I respond with an Action spell or must I use a Reaction spell?

Ruling: When you pass focus and your opponent plays an Action spell, you can only respond with Reaction spells while that Action is on the chain. After that chain resolves, focus passes back to you and you can then play an Action spell.

Sequence:

• You have focus and pass without playing an Action
• Opponent plays an Action spell
• You can respond with Reactions (not Actions) to that Action
• When that chain closes, focus passes back to you
• You can now play an Action spell
• Both players can play Reactions to any Actions played
• Once both players pass focus, combat resolves

Nuances:

• Until an Action card is resolved, you cannot play another Action