Shadow games
.** **shadow games** "battle begin!" "oshawott use scratch!" oshawott sprints toward panpoar faster than one would expect for a small pokémon. cress smiles not looking surprised, "panpoar use double team."
Yugioh Dark Duel Anthology: Chapter 1: Forbidden Memories Reawakened
Solving the puzzle broke the seal on the dark shadow games, and will once again threaten to consume the world.
Hybrid Yu-Gi-Oh Fanfic Season 9, Episode 33
As you may already know, duel monsters started 5000 years ago where shadow games were played for high stakes. however, there's indications that three duelists existed even further back in an era known as the dawn of the machines.