The Dagger, a mortal made weapon of power on scale with Amoriacus, being in the hands of an immortal fae could be what's rare though.
One thing most all immortals we've encountered have either lacked or personified is soul, or any of it's derivatives(i.e. mortality, choice, creative magic, ect) The Dagger I think actually changed the... 'alloy' of Lea (based on the mantles being derivitives of angelic wholes) and gave her a bit of, if not soul 'lively' magic.
Directly when Harry is being saved he thinks he see's an angel in the smoke, with wings. He later discovers it was Lea who saved him though?
So why Did she Manifest wings? because using mortal magic through the knife changed the memories of love she acquired from Susan into part of herself. This is why she acts almost loving toward Harry in SK. Mortal influence gave immortal 'choice'/change.
And No, this isn't the only time this has happened, though one i'd like to point to directly is Lash. Harry basically imprinted a piece of his own make up onto her and she ended up killing herself, much as Harry later does as per Woj. Elaine nudged Aurora down her path over ten years of constant exposer, ect.
Mortal Madness Indeed.
This would mean all Humans are susceptible, but with freedom of will being a main part of them it can't really do more than whisper in their heads. Which if I might harken to my Lilith/Nemesis replaced Lucifer as the Adversary theory. Would literally give her the role(currently mind) we classically give to the father of lies as the whisperer. The shadow on everybodies shoulder, the dark mirror of ourselves...
*a description of Christ's action "removal of an evil force from your spiritual being present in all of humanity".
Al la Lucifer, Now Nemesis. Lucifer took all the negative's he worked with and made their own nitch in reality for(same way Mab did with the Ice Giant's).