Credit goes to MathewMii, Thanks to Poke5869 for Sandayu

Atem is one of the best fortune tellers being cryptic yet just about accurate. Unfortuately, some don't like their such as Sandayu forcing Atem into telling a new fortune, however he'd have to play one of his Pamp Shadow Games, he wins, he'll tell him how to avoid his fate, losing and he has to become a diaper for a few hours.
Atem used his crystal ball to send them into his own little room in the underworld (similar to Yuri's room when someone buys his Pamp God voucher). When they arrived in the underworld, Sandayu felt his Dark Plate drastically weaken, and seeing a flashing before seeing Atem's alter ego, Yami Atem. This made Sandayu uneasy at the sudden shift from neutral to chaotic and evil.

Atem tore off Sandayu's mask and turned into a card and held onto it claiming he'll give it back when the game's over. The game was navigating through a maze while blindfolded, and the maze was completely random that even Atem couldn't navigate it. If Sandayu got to the end, he'd find his new fortune, if he fails than Atem turns him into a diaper. After several hours, Sandayu activated his sharighann to cheat and made it to the end of the maze. When Sandayu flipped the card, it was a joker, he turned to find Atem behind him saying he knew he cheated as the selfish and greedy always cheat to get their ways. Sandayu tried to fight, but found his skills useless as the underworld runs on it's own malice.
"The door to Darkness has opened" and Sandayu soon found himself back the living world, but as Atem's diaper. He taunted him that he shouldn't have cheated, but eventually they always do as Atem waved Sandayu's mask card around. Atem said it would las t a few hours, but would enjoy the torment before leaving him somewhere with as messy diaper while leaving both his mask and a joker card for his lackeys to find since he can easily infiltrate Hisui without anyone knowing.

End

Was in a huge yu-gi-oh mood and decided to show off Atem's dark side, which only appears for his shadow games in the underworld, while the neutral retreates into the millenium puzzle, Atem is more dangerous and fueled with malice in this form as it's a huge personality shift. If you lose than he goes light on the punishments, if you cheat, they get very severe. The Joker card is always used for cheaters or losers, when Sandayu reforms, he needs to tear up the card to get it back.

Atem belongs to me
Sandayu belongs to Poke5869
Pokemon belong to Game Freak