The Nightsong is an integral part of the plot of Baldur's Gate 3 — Halsin gets captured by the goblins while seeking out the Nightsong, setting into motion the events of the game's first act. The game's second act then centres around the party finding the Nightsong beneath the Shadow-Cursed Lands.
However, the Nightsong wasn't always Dame Aylin (technically), the righteous Aasimar we all know and love. Instead, the original plan for the Nightsong appears to be something more Sharran, and less Selunite.
This is evidenced by the name of Lady Shar's model in the final release of Baldur's Gate, which, as discovered by Opening-Secret8217, is labelled nightsong_old.
You can encounter Lady Shar herself during the conclusion of Shadowheart's personal quest.
Ketheric Thorm previously worshipped Lady Shar before pivoting to Myrkul to revive his daughter Isobel. The Nightsong was originally a chosen of Shar, and was perhaps incarcerated by Thorm after he betrayed his patron. Dame Aylin ends up powering his immortality, but perhaps a Sharran originally filled this role.
If we look at cut content for the Nightsong and Dame Aylin, we can glean that the Nightsong was originally intended to be a boss fight with...
