You could literally evacuate everyone from the Last Light Inn in Baldur's Gate 3.
You want Shadowheart to become a Dark Justiciar, thus you are playing Baldur's Gate 3. Killing the Nightsong would thus be necessary. The issue is that everyone of the Last Light Inn dies as well. Wait, though; there's a trick here. Everyone from the Last Light Inn could be evacuated actually!
The shouldershrugger player discovered that moving everyone to the Gauntlet of Shar would save everyone. That dungeon lies just across the Shadow-Cursed Lands. Everyone at the Last Light Inn dies when Shadowheart kills the Nightsong—because of the curse. The curse lets the evil into the inn, so weakening the magic keeping it safe. Moving everyone to the Gauntlet of Shar will thus help them to be free from the curse.
Here's how to accomplish it:
Initially, unlock the fast travel points at the Gauntlet of Shar and the Last Light Inn. Then, at the Last Light Inn, knock off everyone. Use caution! Not kill anyone. Just employ nonlethal strikes.
Move everyone to the Gauntlet of Shar once you have them knocked out. Playing the Dark Urge, the shouldershrugger picked up every character and moved them using their Improvised Weapon ability. Should you lack that ability, you could be able to employ the Throw action.
You can now return to the Shadowfell kill the Nightsong, make Shadowheart a Dark Justiciar, then resume your game. However, you will have less allies, thus the fight with Marcus could be more challenging.
But regarding the Quests?
You may be wondering whether this trick will mess with every quest. The good news is most of the quests are still accessible! You still can hire Jaheira, for instance. Additionally you can keep the Ironhand Gnome questline open and call them allies in the last fight.
The sole issue is that if you apply this trick Alfira might not reach the Last Light Inn. It's difficult to know what happened to the shouldershrugger playing the Dark Urge since she typically kills Alfira. But Lakrissa left Act Two, thus Alfira most certainly will also. We shall, however, have to wait and see for sure.
This trick is therefore perfect if you wish to make Shadowheart a Dark Justiciar but do not want to murder everyone in the Last Light Inn! Though it's not absolutely guaranteed to work, it's well worth trying.
