Geisterdamen

The Geisterdamen are spooky ghost-white women, a lost civilization of amazons living near Dr. Mongfish's secret base/summer home, who ride giant spider-like creatures. Folklore claims they steal children and create Revenants. In reality they are the servants of Lucrezia Mongfish, whom they revere as a goddess (for reasons that her family always found a bit confusing). Brought to Europe by Lucrezia's machines, they are now trapped here, and for the last eighteen years they have searched for 'The Holy Child', offspring of their goddess. Now that they have found Agatha in Sturmhalten, they have been given an avatar of their goddess, and they have vanished. Their current machinations and location are unknown.

Taken entirely from the Girl Genius Complete List of Absolutely Everybody!