Act 2 of Baldur's Gate 3 is when everything starts making a little more sense, but it's also when you have to make much more impactful decisions. There are several decisions in this act that will absolutely irrevocably alter your path through the rest of the game, and one of those decisions is deciding exactly what to do with the Nightsong in the Temple of Shar.
In order to get to the Nightsong, you first need to travel through all of the trials in the Gauntlet of Shar. There are three of them, plus two other areas that you need to fight through, one of which features an absolutely massive Devil that you can either talk into defeating himself or end up in a fight with him and his followers, plus a Displacer Beast.
What Happens If You Kill The Nightsong
If you kill the Nightsong, then you won't only be dooming her. You'll also be dooming the entirety of the Last Light Inn. This means that Isobel, Jafeira, Dammon, and Art Cullagh will all die, with Jaheira no longer able to help you out at Moonrise Towers or become a companion, Dammon unable to help you fix Karlach and progress her romance, and Art Cullagh potentially locking you out of the quest to rid the lands of the Shadow Curse.
Shadowheart will also become one of the chosen of Shar, which means that she becomes a lot more distant with you, and makes romancing her a lot harder overall. She'll get a cool spear, but it's not worth it really for how much you lose.
Saving The Nightsong
If you do choose to save the Nightsong, then you're faced with fighting Balthazaar the Necromancer and his hordes of undead. The issue is that if you do this, you're left on the back foot due to the fact that there are so many of them and you're positioned right next to the edge of an abyss that they'll just kick you down. If you want to save the Nightsong, kill Balthazaar in the room that you find him in during your journeys through the Temple of Shar so that you don't have to face him down in that pit.
Saving the Nightsong will turn Shadowheart away from the path of Shar if you pass a DC 21 Charisma check. While this initially causes her trauma, it's better for her in the long run, plus it means you have a sweet Aasimar absolutely beating the hell out of Ketheric Thorm when you go to Moonrise Towers to face him.