Can you use a reaction spell immediately after your own action spell in a showdown, or does your opponent need to react first? Also, can action spells be used as reactions once the chain starts?

Ruling: You can use reaction spells in response to your own action spell before passing priority. Action spells can only be used to start a chain and cannot be used as reactions at any time.

Sequence:

• Play an action spell to start the chain
• You may hold priority and play reaction spells in response to your own action
• Once you pass priority, your opponent can respond
• After the chain resolves, your opponent has priority to activate an action or reaction before you can respond

Nuances:

• Reactions can be played like actions when the chain is empty (you don't need to react to something)
• Unit abilities with the reaction tag can be activated any time you could play a reaction, whereas normally unit abilities can only be activated during your turn outside of showdown