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Image by Nintendo via Bulbapedia

League Cards are more than just fancy foil-lined trading cards, though it's definitely fun to browse over the collection from time to time

This is a franchise in a perpetual Peter Pan-like state of adolescence

Picture taken from Deviant art Updated answer based on wikia summary It's all explained in this wiki, see the summary section on that link

[spoiler=Wrath of the Gods] If "Obelisk the Tormentor", "Slifer the Sky Dragon", and "The Winged Dragon of Ra" leave the field at the same time: Banish all cards on the field, face-down

Since it has battle destruction resistance and is level 10, it becomes a strong wall that can be attacked in combination with Mound of the Bound Creator