Act 2 of Baldur's Gate 3 picks up the pace. After fighting your way through the Shadowlands and delving into the Gauntlet of Shar, the source of Ketheric Thorm's immortality is revealed, and it's up to you to either kill the Nightsong or wield her as a weapon of light. If you have Shadowheart in your party, this story arc has massive implications for her—do you want her to stay a follower of Shar, or turn her to Selune?
You'll need to make sure you've obtained the Spear of Night from the Silent Library and all four Umbral Crystals in the Gauntlet of Shar before you can access the final part of this dungeon, where the battle takes place. The choices you make will have massive consequences going forward, so beware of spoilers for this section of Act 2. Here's everything you need to know.
Balthazar strategy (easy)
If you've left Balthazar alive to help you discover the secret to Ketheric Thorm's immortality, he'll show up here to take the Nightsong back to Thorm. There is, however, a very easy way to beat this fight: ambush him using stealth before he can get monologuing. Instead of starting a dialogue, simply sneak your party behind Balthazar (shift + C on your keyboard) while he's distracted and attack him.
If you have a Paladin in your group, you might be able to smite him to death during the surprise round, as he only has a measly 70 HP. If your damage isn't up to snuff, however, there's another layer of defence: Counterspell.
If you just talk to Balthazar, he'll open with an uninterruptible Animate Dead, which summons an entire horde of skeletons that turn the fight from a butchering into a genuine boss battle. If you ambush him, however, he'll cast Animate Dead in full view of your spellcasters, letting you Counterspell him.
Granted, his Animate Dead is a 6th level spell, which means your caster will need to succeed on a difficulty 16 ability check to counter it. Quicksave beforehand in case you need a mulligan—this is a cheese strategy, I won't judge you. If you pull this off, Balthazar turns from a fierce adversary into a complete chump.
Balthazar strategy (hard)
While it's a very D&D approach to simply jump the DM's big bad while their back is turned, you might want to take on the fight as 'intended'. Whether you want that juicy story, are roleplaying as an honourable warrior, or just like a hard boss fight, letting Balthazar cast Animate Dead turns the fight into a proper brawl.
Having both a Paladin and a Cleric makes this fight a lot easier. While Balthazar can't be thwarted with Turn Undead, his minions can be, and Divine Smite's bonus d8 against undead still applies to him.
Balthazar is the biggest threat, so your number one priority is to run him down and kill him. He has an aura that makes his skeletons resistant to physical damage, an arsenal of spells like Ray of Sickness that can reduce your damage, and he's shockingly fast for a wizard. Thankfully, he's very fragile and will go down to a couple of turns of focus fire.
If you have a Cleric, keep his skeletons busy with Channel Divinity: Turn Undead. He'll summon mages, destroyers/defenders, and a bunch of footsoldiers. The mages are the biggest threat after Balthazar, as some come equipped with a Sleep spell that forces you to spend one of your four actions per round to shake your allies awake. Don't stand near the ledges, as the destroyers/defenders will try to shove you off for a one-hit kill.
Once you've put Balthazar down, it's just a matter of mopping up the field, then deciding what to do with Shadowheart and the captive Nightsong.
Shadowheart and the Nightsong choices: to Shar or not to Shar
First things first, if you let Balthazar take the Nightsong back to Thorm, Shadowheart will abandon your group. The Nightsong needs to die for Lady Shar, so to let some necromancer run away with her is a massive betrayal.
The first conversation options here are misleading. If you want to save Shadowheart from Shar, you might feel tempted to argue with her right away. However, none of these options actually seal the Nightsong's fate, and pushing too hard will make things worse.
The persuasion check option "No it's not. Your life is what you make it. Don't give up on yourself" has a difficulty of 30—as Gale might put it, it is to be avoided. This check is impossible to beat without maxed charisma and persuasion expertise, bardic inspiration, or a natural 20. If you keep pushing after failing this check, Shadowheart will turn on the party, and you'll be forced to kill her.
Instead, take a breather and respect her initial decision. The Nightsong will keep her talking, getting through to her and giving you a golden opportunity to turn her back to the light. The persuasion option here has a much more manageable difficulty of 21. Succeed, and Shadowheart will throw the Night Spear into the void (don't worry, you get a much cooler version later).
As with any persuasion check, casting Enhance Ability (Eagle's Splendour) before entering dialogue is a great way to ensure success.
Alternatively, you can let Shadowheart kill the Nightsong. This has some gnarly consequences for the people back at the Last Light Inn, however. After driving her weapon home, Shadowheart will become the leader of the Sharran church and a Dark Justiciar, with a killer set of armour to match. However, slaying the daughter of Selune causes her magic to falter, dooming everyone at the Last Light Inn.
While you might be tempted to kill the Nightsong and remove Thorm's immortality, rest assured that there are no negative consequences to freeing her in Act 2. Thorm is feeding off her power, so freeing the Nightsong breaks that connection and earns you a powerful ally.