Act 2 in Baldur's Gate 3 starts when you reach the Shadow-Cursed Lands and unlock the achievement "The Plot Thickens," which signifies leaving Act One for a darker region in the game. The first main location in Act 2 is the Last Light Inn. There are two main ways to get to Act 2:

• Taking the elevator in Grymdark, which is accessed from the Underdark Beach near the Duergar Camp. This route offers more content and better leveling opportunities.
• Going through the Mountain Pass, which is a more direct but tougher path blocked by a difficult fight.

The game prompts you to wrap up any remaining activities before officially transitioning to Act 2. After entering the Shadow-Cursed Lands, the journey through Act 2 continues the storyline with a focus on Moonrise Towers and the Cult of the Absolute. In summary, Act 2 begins when you move into the Shadow- Cursed Lands, and you get "The Plot Thickens" achievement as confirmation. The preferred route to Act 2 is through the Underdark and Grymdark elevator for a richer experience, but the Mountain Pass also leads there.