Before you enter into Act 2 of Baldur's Gate 3 you're presented with a choice. You can either take the Underdark route to get to the Shadow-Cursed Lands or go through the Mountain Pass, though doing both is recommended if you want to get the most out of the game overall. In the Underdark route, you'll discover a True Soul, somebody else who has a parasite like yours, and you'll be given the task of freeing him.
How To Deal With True Soul Nere
You can choose to side with True Soul Nere once you've freed him, though this is very obviously the wrong thing to do. He's a vicious slave owner and an awful person, and his freedom will make the lands of Faerun worse for everybody else. If you choose to fight him, however, it can be a really tough fight and one that feels fairly heavily balanced against you. There is a way to make this fight easier, however.
As you enter into the Grymforge, talk to Elder Brithvar who is just outside the chamber where Nere is trapped. You can convince him to make you an offer which will result in Brithvar and several of the other Duegar stationed here to rebel against Nere once he escapes from the cave-in, which balances the fight out a lot. Firstly, however, you need to talk out the Scrying Eye around the area, which you can do by just stabbing it and then passing a check to convince the Duegar that you meant no harm by it.