Hi, I have a question about Frozen Fortress and the timing of Hidden cards. I control Frozen Fortress as my battlefield, and my Brynhir Thundersong is holding it. I previously played Temporal Breach face down as a Hidden card on Frozen Fortress. On my opponent’s turn, Frozen Fortress’s effect would deal 1 damage to Brynhir Thundersong. Can I play Temporal Breach from Hidden in response to that damage, before it resolves? Specifically, does Frozen Fortress’s damage effect go onto the Resolve Panel, creating a Reaction window in which I can play Temporal Breach?

Yes — the Frozen Fortress ability is a triggered ability, so it goes on the Chain as a Pending Item and creates a Closed State where you get a priority window, and Hidden Temporal Breach has Reaction by then, so you can play it in response (Rules 383, 402.1, 811.1.b).

Two things worth knowing before you commit, though. The Breach doesn't actually dodge the damage: it's still on the Chain beneath your Breach, and when it resolves, "deal 1 to each unit here" isn't a targeted instruction — it just hits whatever units are at Frozen Fortress at that moment. The replayed Brynhir must be played to that same battlefield (it was played from facedown, CR 811.1.d.5), so she's back "here" and takes the 1 anyway. She has 5 Might, so 1 damage is never lethal in the first place — the realistic reason to Breach her here is that she comes back as a new object and re-fires "When you play me, opponents can't play cards this turn" on their turn.

The requirements for the line itself: the Breach must have been hidden on a previous turn (Hidden gains Reaction only "beginning on the next turn," Rule 811.1.b/811.6), and since it's played from facedown, its target must be a unit at Frozen Fortress — Brynhir qualifies. Note this only works because Frozen Fortress triggers; if the damage happened with no Chain item created, there'd be no window at all.