A place of legend, said to hold a weapon able to 'kill a god'.
The door is made of an unknown deep sea green stone, made of cubes bearing unknown symbols and characters.
In truth there are more than one vault, with each serving as a cache for unknown technology, or resources. The caches themselves exist in a pocket dimension with protection against unauthorized entry. The lock itself is actually the entire door. The characters on the door, are oddly incapable of being translated.
The doors may appear and disappear at different places across the galaxy, maybe even the universe. Due to the fact that they disappear and reappear, many believe there is only one vault.
Built by an unknown race, these vaults have become the stuff of legend as no one has been able to successfully open them, before they disappear for another location. Many legends have risen up around the doors, with the most widely accepted that they hold advance weapons technology.