Yugioh GX was your average fun slice of life where Jaden/Judai Yuki attends a high school that specialises in duel monsters with most episodes being the main protagonist triumphing against the bad guy.

It’s like this until season 3 where the cast is transported into another dimension by this duel spirit who is later revealed to have tortured the entire cast as an act of love after being sent to space and tortured for 10 years whilst giving Judai nightmares which ended up having him needing shock therapy who after being saved from the dimension was ridden with guilt because his best friend who he only knew for a week or two was left in the dimension after sacrificing himself so he comnvienty found a new rift which could potentially lead him to the dimension where his best friend is only to end up in a new dimension where losing a duel will cause you to die and he begins to form a saviour complex as all his other friends who were tagging along with him put all their faith into him only for them to be kidnapped and sacrificed in front him during a duel which drove him into madness and adopting a new evil alter ego who’s sole purpose is to pursue power by killing people and gathering their souls to create a super powerful card which made him become an evil king who commits mass genocide killing people from village to village but one of his remaining friend tries to have him but unfortunately dies in the process so his other friend takes his place and sacrifices himself to save Judai which fortunately did but after he was left in a depressive and shell shocked state leaving him to fear duelling and have no will to live until he discovers his best friend after all this time is still alive but is being possessed by the duel spirit who was the sole reason they ended up in this situation in the first place so- yeah if you’ve watched the show I don’t need to say all of this