Before heading to Nightsong in Baldurs Gate 3, it is recommended to complete as many side quests and objectives in the available areas as possible. Entering Nightsong is a major point of no return in Act 1 of the game, and it removes most quests in Act 2 such as rescuing the tieflings/gnomes and finishing the quest after Nightsong, which locks you out of Act 2 entirely. Here are some of the quests that should be completed before heading to Nightsong:

• Complete all Moonrise quests.
• Complete Halsins side quests to save Thaniel.
• Recruit Jaheira the Harper.
• Get rid of the entire Thorm family.
• Rescue all the prisoners in the Moonrise Towers.
• Capture/save Isobel the Selunite Cleric.
• Infiltrate Moonrise Towers.
• Find Ketheric Thorms relic.
• Follow the convoy.
• Resolve the abduction.
• Seek protection from the Shadow Curse.
• Defeat Ketheric Thorm.
• Find Zevlor and Mol.
• Find the Nightsong.

It is important to note that completing the "Lift the Shadow Curse" quest before completing Nightsong is recommended, as it can really mess up some other quests which require protection from all but the darkest shadows. Additionally, completing the "Find the Nightsong" quest will automatically resolve some other quests, so be sure to complete those quests before completing this one.