Darknite

Darknite is a villain that appears in some of the Yugioh games, but not in the main series. He serves as a Bigger Bad in every case. In the Japanese version, he is called Card Majin, which (in Japanese culture) a Majin is a malicious deity that causes disaster with it's mere presence. He could also be considered one of the most evil (or at least the source of it) entities in the series, due him creating the Shadow Games.

Darknite can also transform into a more powerful form known as Nitemare.

Yu-Gi-Oh! Forbidden Memories
Darknite is sealed away in the Forbidden Ruins and, as stated above, is famous for the creation of the Shadow Games, which when one loses, is sent to the Shadow Realm. Darknite can only be released when all seven of the Millennium Items are brought to his tomb.

Heishin brings all of the relics to the ruins to awaken him. Once there, Heishin attempts to make a pact with Darknite to help bring him higher to power. However, Darknite expresses no interest in the bargain. Heishin pleas with Darknite, however he grows pestered with the conversation and turns him into a card. He then comments that it is "the most noisy and ugly card he had ever seen". He then burns the card (likely killing Heishin) saying that it doesn't belong in his deck.

Darknite then faces to Atem and Seto who had seen this entire event and claiming they would be excellent additions to his deck. Atem then reveals the Millennium Item Cards to Darknite, forcing him to back off. However, if doesn't last long and ultimately Atem must duel Darknite. After losing to Atem, Darknite becomes extremely enraged and transformed into Nitemare, demanding another duel with a far more powerful and impressive deck. Despite this however, Atem defeats Nitemare again, which sends him back into where he once was.

Yu-Gi-Oh! The Falsebound Kingdom
Darknite serves as the final boss and is released by Scott Irvine after Kaiba and Yugi destroy his virtual world. Darknite proclaims that "the price for interfering with a minion of darkness was high" and tries to swallow the souls of the hero. Darknite fails, and battles them with one of the most powerful monsters in the entire game: An Egyptian God (varies on who you play as). After beating Darknite, the virtual Kingdom will shut down forever.

It is possible to face Darknite again, and he will transform into Nitemare with his Egyptian God having maxed-out stats.

Yu-Gi-Oh! Dark Duel Stories
Darknite/Nitemare is the final boss of the handheld game Dark Duel Stories. His role as an antagonist here is downplayed and seems intensification. His only purpose to "eat the player's soul" because he has proven to be the best due to the player succeeding in things even he couldn't do. Besides that, he also has some of the most powerful cards in the game.