Mount Tantiss was originally the site of an Old Republic fort that was reactivated by Palpatine during the Clone Wars. The storehouse contained both objects and trinkets of unspeakable evil, as well as mementos from the conquests of the Empire. His most private treasures were hidden in a massive labyrinth designed by Garrbo V’Droz in the mountain’s bottom chamber.
The mountain and its storehouse were guarded by a Dark Jedi referred to as «the Guardian» until he was (presumably) killed by the insane clone Joruus C’baoth.
The mountain also held the twenty-thousand Spaarti cloning cylinders which were used by Grand Admiral Thrawn to create a crew of clones for his new fleet of Dreadnaughts, and the schematics for the cloaking devices that he would install on those very same ships and would make their way into much of the Imperial Fleet as time went by.