Unknown Event 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, Each player opens a Magic the Gathering booster pack. They may select up to two cards from that pack and exile them face down. For the rest of the game, players may look at and play cards exiled this way without paying their mana costs. (Then Planeswalk away from this phenomenon.)