Instead of a banshee nearby to give the party clues, I might have this abandoned city be retaken by a something more suitable for this setting: A Green Hag. She’ll be reintroduced the same way: Garaele, the Harpers priestess from Phandalin, will give her that silver comb for the location of that tome (which may, in this setting, lead into a campaign of its own).

This hag would be my twist on the traditional role of Hags, a powerful witch who, while more than apt to make bargains and demand that the recipient's part be played, is less Evil than you’d she's more apt to set up her own coven teaching the younger generation the craft. She usually takes up orphans to warp them into her image, but doesn't go about kidnapping babies to eat them. I’ll use my modified Witch class to create her, and keep Agatha for her name. It fits her well.