There's references to Netheril a few times in the EA, including a book on a ruined city, I don't know enough about FR lore to earnestly work it into a good theory.
The Hag explicitly says it's "Netherese Shadow Magic", very setting specific terminology here. Shadow Magic in FR is magic created by the Shadow Weave. Yes, up untill now I don't think we have many references in 5e besides the
Fallen Floating City in RotFM,
but one we certainly have is that the main Puppeteer behind the events of 5e is indeed
Vecna, using the ancient Netherese obelisks throughout all the 5e adventures as some kind of "Pocket-Time travelling devices".
I wouldn't be suprised if we come across such stractures further in the game.
Anyways, one thing or the other, Shar is definitely aware of the tampering with the tadpoles and I wouldn't be surprised if the
God of Evil Secrets has tampered his hand into this one. As outside marketing goes Vecna is getting really pumped up as we speak: revamping, figures, new printings, appearance in Stranger Things etc...
