From the Deck of Many Things (DMG, p. 164):
Throne. You gain proficiency in the Persuasion skill, and you double your proficiency bonus on checks made with that skill. In addition, you gain rightful ownership of a small keep somewhere in the world. However, the keep is currently in the hands of monsters, which you must clear out before you can claim the keep as yours.
It says "somewhere in the world"; does this mean on the Material Plane, or could it potentially be on any plane of existence? I know it's left intentionally vague to allow the DM more flexibility, but I wondered if "the world" at least implies the Material Plane...