firstly, youll need a Leader card, and you can only use cards in your deck which have the same color as your Leader card (see all of the six card colors shown above, as they appear on each card)
The same thing is true to Atris when targeting multiple people
Due to most of the "Millennium" monsters have an effect that require the user to pay their Life Point costs, or losing the LP as a maintenance cost in the case of "The Unstoppable Exodia Incarnate", player's LP will be quickly reduced in the prolonged game stage, especially if they doesn't have "Millennium Ankh" in their hand, thus leaving them vulnerable from final damage push by the opponent
The anime pushed it further with Ash-Greninja, and Smash Bros put it in front of an audience that doesn't even play the card game
Since it only takes ten poison counters to eliminate a player, it's an effective way to take out two opponents at once with only a few creatures in play