Jaina, Lor’themar, and Thalyssra were all stumped as to what type of magic Sylvanas was using. Many people infer that it’s void magic, since it’s a dark smoke very similar to what N’zoth uses. It could alternatively be necromantic magic though, as the spell didn’t at all wound Saurfang so much as deliver instant death, and his body went instantly pale.
I’ve also read a theory that Sylvanas is a dreadlord, making this fel magic, which is usually green but can be black/purple. This would mean she’s kept this magic hidden.
The only reason this doesn’t seem to be the case is the dialogue after the fight shows that neither Jaina nor Thalyssra (idk how to spell her name) recognized what type of magic she was using. I don’t remember what they said exactly, but maybe something like “they couldn’t sense it” or something.
They’ve likely been around shadow magic enough to recognize it, so shadow is likely off the table.
No its not shadow magic, its death magic , instant death caused , Sylvanas is going to kill literally everyone in the next patch , but then resurrect us at the same time to be our true lord and saviour, its to prevent N’zoth from absorbing us.