This huge and glorious mansion is owned by the revered trader and some say noble lord Willem Ostheimer.
Its ballroom and gardens are famous, and bring in visitors regularly. It hosts an annual party – the Marienburg Masked Ball – invites are sent to many people of note, many times just before they became notable. Invitations are rarely sold, but when they are it is for a significant sum. Adventurers and explorers are regularly invited, as their stories liven up the evening.
The mansion was until recently owned and managed by the vampire noblewoman Elouise, and traded to the party for a mission (episode 27). Willem Ostheimer doesn’t exist and up until recently has been one of Elouise’s servants in a mask. Upon receiving the mansion the party have replaced all of the serving staff.