Yu-Gi-Oh! started as a manga by Kazuki Takahashi. It's about a boy named Yugi who likes to play games and solve puzzles. I don't know much about the manga or anime, but from what I gather (because I want to not spoil it for myself), he solves some "Millenium Puzzle" from Ancient Egypt, and becomes possessed by the spirit of a Pharoah who likes to gamble and takes over when Yugi and his friends are in trouble an challenges their enemies to a duel using the card game that wold later be created irl, and the creatures in the cards actually come to life and actually battle in the world of the manga. And that is fcking awsome!!!!!