The Great Library Heist

Just before the Brothers' War, a team of unknown actors reportedly stole hundreds of magical artifacts and books of knowledge from the Blood Library. No one but the Blood Archivist knows if this really occurred, as the Archivist immediately locked down the Bloodless Citadel and declared an end to diplomacy with fickle, fleshy mortals. None of the diplomats, scholars, and merchants in the Citadel were ever seen again.

No one knows who the thieves were, but most people will blame/praise the Thieves Guild, while guild legends blame agents in the Magocratos.

Most vampiric items and forbidden tomes can be, if their owners are to be believed, traced to this heist.