Nitemare

DarkNite is the main antagonist in a few of the Yu-Gi-Oh! video games. He appears in Yu-Gi-Oh! Dark Duel Stories, Yu-Gi-Oh! The Falsebound Kingdom and Yu-Gi-Oh! Forbidden Memories. He is capable of transforming into a more powerful form known as Nitemare.

Kazuki Takahashi included lineart for Nitemare, along with other rough sketches for cards for the real life card and video games in Yu-Gi-Oh! Duelist volume 11.

Yu-Gi-Oh! Forbidden Memories
In Yu-Gi-Oh! Forbidden Memories, DarkNite is known to be the creator of the Shadow Games. He is sealed away in the Forbidden Ruins, only to be awoken when the 7 Millennium Items are brought there.

When Heishin awakens DarkNite, he attempts to order him claiming it is part of the pact, but Darknite says that he does not recall making a pact with Heishin. When Heishin begins to plea with DarkNite, he quickly grows tired of the conversation and turns Heishin into a card. Finding it to be nosiest and ugliest card he ever created, DarkNite burns it saying it doesn't deserve a place in his Deck.

DarkNite then turns to Atem and Seto saying that they would make nice additions to his Deck. Seto reminds Atem about the Millennium Item Cards. Atem then holds out the cards forcing DarkNite back. Unable to fully hold him back, the prince is left to duel DarkNite. After losing once DarkNite refuses to accept his defeat and turns into Nitemare demanding another Duel. Atem proceeds to defeat Nitemare, sending him back to where he came from.

The Falsebound Kingdom
In Yu-Gi-Oh! The Falsebound Kingdom, DarkNite is the final boss. He is described as a "dark spirit," who is turned loose by Scott Irvine after the hero (either Yugi, Seto, or Joey) dismantles Irvine's virtual world, The Kingdom.

Though DarkNite has only one monster, it is one of the powerful Egyptian God Cards. Which god card is faced depends on the hero, and seems to correspond to the God each hero has the most rivalry with; Yugi faces "Obelisk the Tormentor," Seto faces "Slifer the Sky Dragon," and Joey faces "The Winged Dragon of Ra." After defeating him, The Kingdom is shut down once and for all.

On a replay game, it is possible to face him as Nitemare, whose God card will have maxed-out stats.