The cut scene where the voice of the absolute forces the party to its knees and SH's artifact drives it off. This happens just before the overland map is revealed.
We can get to the end of Act 1 really quickly, if we want; skip the grove, circumnavigate Moonhaven, then bypass goblin guards. Possibly start of mountain pass and Moonrise is Act 1 too?
But we don't really know whether Moonrise Towers or Nightsong Temple are Act 1 or 2. I would wager they're Act 2, which we encounter early but can't resolve until later. (If I were the TT DM, that's what I would do, introduce the unkillable BBEG, party suffers a bit, does some questing, then get the big pay off when they take him down.)
