The Silvermote Company
A magical extra-dimensional abode anchored in Waterdeep’s Noble ward, with entrances in several other areas of the city (and rumors of some outside he city). The Silvermote charter of the allowed use of extra-dimensional space in this way goes back at least as far as the founding of Waterdeep, possibly earlier.
In order to enter the manse, you need to be invited by someone of Silvermote blood, or you need to be bearing a Silvermote Pendant, which was freely given in ceremony called tainspinning.
The manse serves as the headquarters of the Silvermote Company, and contains a small towns worth of storage, personnel, living facilities, and amenities.