Can you cast a spell with flash during your opponent's end step after they pass priority?

Flash (CR 702.8) allows a spell to be cast any time you could cast an instant. Instants can be cast whenever a player has priority, including during any phase or step of any player's turn — including the opponent's end step (CR 307.1, 116.1).

Priority passes around the table after each spell or ability resolves, and also at the beginning of each step and phase. When your opponent's end step begins, your opponent receives priority first (CR 116.3). If they pass priority without doing anything and the stack is empty, priority passes to you. At that moment, you may cast a spell with Flash.

It is important that the stack be empty when your opponent passes, or that you are responding to something already on the stack. If your opponent casts something in their end step, you can also respond to that with your Flash spell, since you would receive priority after they finish casting it (CR 116.3c).

Example: Your opponent is at 3 life. They pass priority in their end step with nothing on the stack. You cast Briarbridge Tracker (which has Flash) — perfectly legal. It enters the battlefield before the end step ends.

One small caveat: some spells have additional timing restrictions or require specific game states (e.g., only when a creature attacks). Flash overrides the default sorcery-speed restriction but does not override those extra conditions (CR 702.8a).