The Hall of Mental Splendor was a small keep in the city of Skullport in Undermountain that was the headquarters of an organization of psionic mercenary spies led by Vhondryl active in the 1370s DR.
The psychic spies of the Hall of Mental Splendor conducted espionage for those who hired them, promising discretion. They worked for many of the feuding factions in Waterdeep, the city lying in the world above, but Vhondryl persisted in seeking clients and work outside both cities. She recognized that every job could create an enemy, and would rather keep them far away and not directly above.
Their work could include surveillance of certain persons, or sabotage of their efforts. They might hire subcontractors, particularly for missions well outside the city of Waterdeep.
The highest-ranking and most capable agents of the Hall were mostly cognition thieves, psychic spies who could infiltrate a person's very mind and steal their thoughts or their wits. All cognition thieves in the Realms were trained at the Hall of Mental Splendor, though some later retired or otherwise parted company.
Around the beginning of Mirtul, the Year of Rogue Dragons, 1373 DR, a mission went very wrong and an argument over it erupted between Vhondryl and the doppelgangers. The doppelgangers quit, leaving the Hall without any shape-changing capacity. Vhondryl began looking for potential agents who could shape-change through magic such as polymorph or even just had excellent skills at disguise.