The World of Darkness is a realm where humans are tortured by constantly being forced to live visions of their dreams being crushed until they give up on their individuality, and whoever is sent there is erased from the memory of others. It resembles the Shadow Realm, but isn't the same thing.
During the fourth season of Yu-Gi-Oh! GX, Trueman and Nightshroud pull several people, including Sartorius, Sarina, Axel Brodie, Tyranno Hassleberry, Taigo Sorano the many residents of Domino City, all the students at Duel Academy (save Jaden Yuki), and eventually the whole world, into the World of Darkness. Jaden, however, is able to get all of the victims to focus on holds their dearest memories, allowing them to escape.