Can an opponent react with hidden cards (cards from hand) when Brynhir's ability resolves, and does this create a chain reaction?

Ruling: Yes, Brynhir is a triggered ability that gives the opponent an opportunity to react before it goes into effect as it resolves. However, once Brynhir fully resolves, the opponent cannot use any actions/reactions from their hand.

Sequence:

• Brynhir's triggered ability goes on the chain
• Opponent has opportunity to react with hidden cards before it resolves
• Once Brynhir resolves, opponent cannot play cards from hand
• Any abilities already on the chain will continue to resolve

Nuances:

• "Playing a card" refers to the discretionary action of playing from hand, and a card is considered played when it has finished resolving
• If attempting a showdown after Brynhir resolves, opponent cannot use actions/reactions from hand