The Millennium Upgrader is an easter egg item. Originally an upgrade kit, it was functionally removed from the mod in 3.2. Its only use is in a chain of item conversions that results in the Experiment W-801.
Like most other easter egg items, Millennium Upgraders do not appear in the creative inventory. They are also unobtainable through /give, as they are instantly deleted when obtained in this manner.
When used by a player inside lava to hit a mob that is simultaneously inside lava, the Millennium Upgrader will turn into the Molten Upgrader.
When used, the entire stack of Millennium Upgraders will be consumed, but the output will always only be one item, so it is a waste to use more than one at a time.
- Prior to being made into an easter egg item, the Millennium Upgrader was an upgrade kit used only to upgrade the Lunar Greatblade into the Millennium Greatblade. In 3.2, the Light Upgrade Kit assumed the role instead.