bg3 when does act 1 end

Act 1 of Baldur's Gate 3 ends when the player leaves the Underdark or the Mountain Pass and receives a message prompt saying, "You are about to progress your adventure. Make sure to tie up any loose ends before advancing!" At that point, after confirming, the game transitions into Act 2. This typically happens after interacting with an elevator in the Underdark or a wooden gate in the Mountain Pass, leading into the Shadow-Cursed Lands. Though players can return to Act 1 areas after starting Act 2, some quests and storylines will be locked or move on based on progress. Essentially, the end of Act 1 is marked by entering the Shadow-Cursed Lands, which ushers in Act 2.