This is a specific type of MacGuffin
that is being sought because it will allow you to rule. Or if you're already a ruler it will allow you more or less unchecked expansion as you Take Over the World
This crosses over with other tropes - usually it's an Artifact of Doom
so powerful none will dare stand against you. However it can instead be more benign, a Cool Crown
or Staff of Authority
which magically confers kingship. The main point is that the story considers a single item essentially a Golden Snitch
that makes your rulership and/or conquest a foregone conclusion.
Do We Have This One?
before I flesh it out?