Hidden
Can a card Hidden at one Battlefield respond to a play at another?
A Hidden card can respond to play elsewhere
RiftPal explanation · Answer first
Actually
Yes, if you have a legal 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 opportunity and can meet the card’s requirements. Where a play happens does not by itself prevent a card elsewhere from responding.
RiftPal explanation
Why it matters
Starting next turn after you hide it, the card can be played with Reaction at normal legal opportunities. That permission is separate from its TargetA specific object, player or zone chosen by a spell or ability under the targeting rules. Check legality when choosing and when resolving. Costs, automatic groups and explicit "pick" choices do not target; pending Chain items cannot be targeted.View in Glossary : Hidden still restricts certain choices to the BattlefieldA Battlefield is a shared location that players compete to control during the game.View in Glossary where the card was hidden.
You do not need an event at that Battlefield to unlock Reaction. You still need 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 , or 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 and Priority with 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 during a ShowdownA Showdown is a structured opportunity for players to act when a Battlefield is contested.View in Glossary , and legal targets.