Royal Booster is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

When you encounter this phenomenon, open a Magic: The Gathering booster pack and shuffle it without looking at its contents. Those cards become the booster deck. You become the monarch. For the rest of the game, at the beginning of the monarch's end step, that player draws a card from the booster deck. They may spend mana of any type to cast those cards. (Then Planeswalk away from this phenomenon.)