Originally Posted by Silver/
Originally Posted by Taril
Originally Posted by ldo58
As a boon for his efforts for Shar, Ketheric gains immortality from Aylin's situation.

Though, it's clearly not Shar who provided the immortality to Ketheric. Since he again binds her to him at the end of the Mindflayer Colony - Which is why you have to free her again to be able to harm him.

Seemingly it's something that Ketheric himself did to Aylin (Likely hinted at with the ritual circle in his room, that's seemingly where he studied the ritual required to link his life to another. Presumably after meeting Aylin and finding out that she is immortal)

Of course, the question is how Aylin is so immortal in the first place. Not even Greater Gods can manifest a form that simply shrugs off all injuries like Aylin (And subequently, Ketheric via the binding) seems to do. Like, normally "Immortality" in DnD involves a body "Dying" and then being sent back to a home plane to recuperate for some time. Not just stand there and heal back up like Wolverine/Deadpool.
It's not an innate trait to herself either, but granted by a passive that explicitly states this trait comes from having an exceptional amount of Selune's favour/having been granted a favour by Selune

https://bg3.wiki/wiki/Aylin

This is somewhat further confusing the in game logic that goes a bit like this:
1. Aasimar are descended from or touched by Gods
2. A direct descendant of a god would be even stronger
3. Aylin is an exceptionally strong Aasimar and Paladin (except, not really).

Aylin could have been an unusually blessed Aasimar, that displays Deva like traits because of a more direct than usual bloodline, but that's not really her title. This is a self proclaimed demigod.

That also means whoever redirected her boon towards Ketheric messed with some extremely high level magic, and although Ketheric is able to cage her at will, he can't make himself immortal again as easily.

This actually reinforces a point I made earlier and I didn't even remember this trait so thanks for bringing it up.

>"Blessed with the favour of a goddess, Nightsong cannot be permanently killed. When unconscious, the start of her turn she recovers 1 hit point."

That settles it then - Aylin doesn't 'instantly respawn' upon death. Even Selune herself wouldn't be able to pull that off. She just recovers from unconsciousness to 1HP like a high-level Barbarian, so she never actually truly dies, at least from the wounds that the spear or Sharrans can inflict on her. That being said, she could probably experience true death at the hands of Arcanists or creatures of higher level/CR than those shown within the scope the game. Power Word Kill would likely send her back to the Upper Planes, as well as a bunch of other creatures who have 'immortal-buster' powers. Technically, Disintegrate should also send her spirit back to to her home planes if working as written.

P.S 'Divine FAVOUR' even further implies to me that Aylin is just a loyal deva and not a true 'daughter' as she tries to convince herself in her dialogue.