Vellum Citadel is a location in Tyranny.
The Vellum Citadel was a library fortress, the largest archive of the written word in the Tiers. The School of Ink and Quill protected and maintained the archive of ancient lore for centuries, standing apart from the political upheavals of the Younger Realms as an enduring pillar of knowledge and culture.
The Sages of the Vellum Citadel publically swore fealty to Kyros, but from within the safety of their Vellum Citadel, these scholars continued to study and copy forbidden lore, and used magic to repulse any attempts by the Disfavored and Scarlet Chorus to claim the Citadel that was promised to the Overlord.
Choosing the Citadel allows you to determine if your Spires will have Sage recruits, as well as the loot composition within the Burning Library. Also, Sirin is heavily involved in the region, allowing you to earn much Loyalty or Fear with her.