Can you use a hidden card (like Fight or Flight) during an opponent's end phase to prevent them from moving on their next turn, or do you need something to respond to?

Ruling: You cannot use a hidden card during the end of turn phase unless there is an item on the chain. Discretionary actions during the end of turn phase require a chain to exist, and cards/activated abilities must have the reaction keyword to be used in response.

Nuances:

• Simply wanting to prevent movement on the next turn is not sufficient reason to activate a hidden card during end phase
• The opponent would have the chance to move again if they're readied (if you could activate it without a chain)