Hopefully this clears everything up;

Originally Posted by Draghmar
...actually at the moment of Dame Aylin's death, there was no mention that she is tied to the Selune in any way.

From the moment one enters the Gauntlet of Shar it is full of books, letters, frescos, statues, companion comments and numerous other audio & visual elements including Shar's own monologues which portray the history of the place, along with the process of becoming a Dark Justiciar by killing a Selunite while implying that the person inside the Shadowfell is a powerful being connected to Selune. Because the whole "Selune vs Shar" thing is the underlying centerpiece of ACTs 1 & 2 stories and the whole reason why ACTs 1 and 2 are in such disarray, with Ketheric leading the entire plot.

  • A book about Nightsong laying underneath Selune's statue in the Underdark in ACT 1;

    [Linked Image from i.imgur.com]

  • Shadowheart herself prior to taking the elevator down says that the sacrifice inside the Shadowfell can't be just a mere mortal.

    [Linked Image from i.imgur.com]

  • The Sharran riddle confirming only Shar can kill the Nightsong (therefore it is absolutely not a mere mortal that has no connection to Selune);

    [Linked Image from i.imgur.com]

  • A fresco stating the person inside is Selune's own blood (because this temple emerged after Ketheric turned to Shar);

    [Linked Image from i.imgur.com]

  • If these clues are too subtle; then prior to entering the Shadowfell Shar herself literally speaks to Shadowheart and asks her to kill the Selunite imprisoned inside it. The player can listen-in on this conversation if they enter Shadowheart's thoughts while Shar is speaking to her.

    [Linked Image from i.imgur.com]

There are countless of lore bits all around the place quite clearly revealing to anyone paying attention that the Nightsong is an immortal Selunite who has been imprisoned by Ketheric and got killed repeatedly by thousands of Sharrans in the last 100 years because all of the Dark Justiciars we see around ACTs 1 & 2 were Ketheric's own army, most of which converted from Selune to Shar so they had to kill a Selunite to become one.

Which is why if you as a player kill Aylin accidentally (or intentionally), your character will gain the [DARK JUSTICIAR] tag and then she'll revive back immediately to taunt you about it;

[Linked Image from i.imgur.com]

Ultimately when she is truly killed because of Shar's unholy spear it is not Shar who suddenly gains some power boost for her curse to overwhelm Isobel, because it is not Shar in the first place who is even maintaining the curse. It's Ketheric who unleashed it and maintains it (confirmed by Halsin, Thaniel, Thisobald Thorm). Instead what happens is Selune's power concentration wavers because her own daughter just got murdered so Isobel can no longer channel enough power from her goddess and everyone dies.

Which is what I said twice, so am not sure where the misunderstanding that she was channeling Aylin's power came from because Aylin's celestial powers are completely dampened as long as she remains shackled to Ketheric.