Showdowns and the Chain
Once a Showdown starts, are only Reactions allowed, or is an Action allowed only as its first card?
A Showdown can contain several Action Chains
RiftPal explanation · Answer first
Actually
During a ShowdownA Showdown is a structured opportunity for players to act when a Battlefield is contested.View in Glossary , an empty ChainAn off-board zone that holds cards and abilities during the process of playing them. Cards and abilities on it are public information.View in Glossary and FocusThe opportunity to act in a Showdown when the Chain is empty. Gaining Focus also gives Priority; you need both to act. Passing Priority during a Chain does not itself pass Focus.View in Glossary let you start another legal ActionA timing keyword that also permits a card or activated ability during a Showdown on any player's turn. You still need Focus and Priority with an empty Chain; Action alone does not allow a response to a waiting Chain.View in Glossary or ReactionA timing keyword that includes Action's permissions and also permits a card or activated ability while a Chain is waiting. You still need the proper opportunity, normally Priority; it does not allow an interruption halfway through resolution.View in Glossary Chain. A Chain is only one part of the Showdown.
RiftPal explanation
Why it matters
While a Chain is waiting, Action alone does not let you respond: you normally need Reaction and PriorityThe right to take a legal action now. Only one player can have Priority at a time, and having it does not remove a card or ability's timing restrictions.View in Glossary . Once that Chain finishes, the Showdown returns to an open state, and the player with Focus and Priority can act again.
Everyone passing Focus in sequence without acting ends the Showdown. Finishing one Chain does not.