Chess grandmaster Hans Niemann is suing one of the sport's biggest legends for $100 MILLION ... because he says the guy made up claims that he cheated during a match.
Niemann played Magnus Carlsen -- a five-time reigning World Chess Champion and the highest-ranked chess player ever -- at the Sinquefield Cup in St. Louis back on Sept. 4 ... and, in Niemann's words, "soundly defeated" him.
Afterward, the 19-year-old budding superstar danced on Carlsen's grave, taunting him during an interview ... and that's when Niemann claimed all of the trouble began.
The chess player, in a lawsuit filed on Thursday in Missouri, said 31-year-old Carlsen couldn't accept defeat, became "enraged" ... and launched an all-out attack on his rival.
Niemann says Carlsen fabricated a story that he had cheated to win the match. Days later, when the two were supposed to run things back at the Julius Baer Generation Cup ... Niemann says Carlsen "gutlessly forfeited the game after making one move" and then repeated the "false" cheating accusations against Niemann.
Niemann says the ramifications of Carlsen's lies were endless.
He claims that despite several independent and unbiased sources clearing him of any wrongdoing during the Sept. 4 match ... Carlsen used his partnership with Chess giant Chess.com to essentially blackball him in the chess world.
He says in the suit that the org. banned him from its website and all of its future events -- which comprise the majority of the world's most prestigious chess tournaments.
Niemann says he's suffered irreparably from Carlsen's defamatory allegations ... and is suing for NINE figures.
Carlsen has yet to comment on the suit. Niemann, meanwhile, tweeted it out Thursday with the caption, "My lawsuit speaks for itself."