The Treasure of Thundera was a cache of Thunderian artifacts, mostly magical in nature. Ratilla tried to acquire the treasure in the past, but Jaga stopped him. When Thundera was destroyed, the chest containing the treasure survived, and when Mumm-Ra recreates the planet, it is found again upon the world.
The Treasure of Thundera has existed for an unknown time during the ThunderCats' history. What is known though is that the Sword of Omens is a part of it's treasures, meaning it's just as ancient as the Sword is. Within the chest is a plethora of treasures entrusted to the care of the ThunderCats. Treasures of various powers and abilities. For these items to be in the hands of evil would spell doom for the ThunderCats, and the rest of the universe.