The Reliquary is a unique Otherworld in that it channels the ideal of a treasure, a powerful, ancient, important treasure that might solve all of its seeker's problems.
It was only opened twice in recorded memory. Once by Percival, a member of the Circle of Merlin, who found the fabled "Holy Grail", and once by Aladdin, Godborn of Bastet, who retrieved a lamp holding Sibal Wilm Rion, a powerful Djinn.