From Destinypedia, the Destiny wiki
SIVA Caches appear after each of the three main bosses in Wrath of the Machine: Vosik, the Archpriest, the Siege Engine, and Aksis, Archon Prime. The chest can be opened via the use of a SIVA Cache Key. The chest will contain an extra piece of loot from the respective bosses loot table, along with a chance for Strange Coins, Motes of Light, the bosses respective mission item, or Exotic gear. Each chest can only be opened by a player once per week. On Hardmode, a second chest spawns containing loot form the bosses Hardmode loot table.
When a player has used a SIVA Cache already before reset, the Cache will read "Disabled"
List of appearances