In Baldur's Gate 3, you spend most of the game fighting against Mind Flayers, giant walking talking Squid men with psychic powers. They're the impetus behind your entire adventure, and continually cause you a ton of problems throughout the entire game. By the time you reach the end of Act 2, you'll be ready to fight against any Mind Flayers that you happen to see on your journey, which makes a certain decision at the end of Act 2 even harder.
Should you side with the Mind Flayer?
At the end of Act 2, just before you actually reach the city of Baldur's Gate, you'll set up camp but will be extremely restless and unable to sleep. You'll then hear the voice of the Dream Visitor that has been helping you throughout the entire game directing you towards a portal that has opened up just outside your camp to help them defend against something.
Once you make it through the portal and toward the source of the voice, you'll make a discovery. That Dream Visitor that you created all the way back at the start of the game, fully customizing it? It's actually a Mind Flayer, but one that is able to think for itself and is seemingly on the side of good.
You're given the option here to either fight the Mind Flayer or side with him, but you should absolutely side with him. That's because if you fight with him and defeat him, the game ends. You don't ever get to Baldur's Gate, and you don't make it out of that portal, instead splitting open and birthing a new Mind Flayer.
Siding with him means that you have to fight several Githyaki Monks that have made their way into the area in an attempt to free a prisoner, but it also means that you'll get more answers as to what exactly is actually going on here, what the Mind Flayer is doing here, and it means that you'll actually get the chance to go to Baldur's Gate itself.