i have heron and thousand tail watcher on a battlefield, my opponent uses rampage paying its additional cost targetting his empowered ambessa, the wolf, and my heron, i play from hidden temporal breach targetting my thousand tail watcher to reduce the might of his units, can i play a spell later with the reduction from heron?

Incorrect timing on the Temporal Breach replayed Watcher pending status and the Heron trigger pending status.

Yes — If Temporal Breach is your first card played that turn, but the discount lands on the card you play after the Heron trigger resolves, not on the Watcher Temporal Breach replays. Temporal Breach is your first card this turn, and it resolves before Rampage, so Heron's trigger fires when Temporal Breach finishes resolving while Heron is still at a battlefield. That trigger goes on the chain, resolves, and your next card this turn then costs [2][A][A] less — even if Rampage resolves afterwards and kills Heron, since the condition is checked when the trigger fires and the pending discount survives removal (FAQ #11829). How the timing works in the rules set: Temporal Breach puts a unit on the chain as pending as part of its resolution. Astral Heron triggers and goes on the chain after Temporal Breach resolves. The chain will be: 1 - Unit: Thousand-Tailed Watcher (pending) 2 - Heron trigger (pending) We finalize up the chain. Chain item 1: the Watcher enter play immediately (units don't linger on the chain). It's 'when you play me' triggered ability gets added to the top of the chain as pending. Chain: 2 - Heron trigger (pending) 3 - Watcher trigger (pending) We finalize 2, the Heron trigger resolves and gives the discount to the next card played after that. We finalize 3, Watcher reduces the Might of all opposing units (this turn). After all this the opponents Rampage can finalize. One practical check: since you played Temporal Breach from Hidden, its target must be a unit at the battlefield where you hid it (Rule 811.1.d.2) — so the Watcher has to be at that same battlefield.