Interesting development, Skyrim mechanics aside, why didn't she make Serana a head shorter and simply take the scroll? Yes, there is the info if dissent in the vampire ranks, but how does she know Serana is on the 'Sane' side? After all, unlike the DB in a 'normal' playthrough, who gets just told to investigate in general, Luxia was sent to retrieve the artifact. Or at least keep it out of vampire hands.
At any rate, can't wait to find out what happens inside Volkihar Castle. ^^

6 minutes ago, Talesien said:

Interesting development, Skyrim mechanics aside, why didn't she make Serana a head shorter and simply take the scroll? Yes, there is the info if dissent in the vampire ranks, but how does she know Serana is on the 'Sane' side? After all, unlike the DB in a 'normal' playthrough, who gets just told to investigate in general, Luxia was sent to retrieve the artifact. Or at least keep it out of vampire hands.
At any rate, can't wait to find out what happens inside Volkihar Castle. ^^

The first reason. Gameplay.
If Serana dies, Dawnguard’s quest remains blocked and impossible to advance.
The second reason. As was said in the previous chapter, Luxia is very intelligent and can feel evil presences or other.
There, she fell in front of Serana, she did not detect any threat coming from her. Even if it is a vampire.
And it will be explained later, why Luxia spared Serana.