Cyberdark

The Cyberdark are a series of three monsters: Cyberdark Keel, Cyberdark Edge, Cyberdark Horn, and their fusion monster: Cyberdark Dragon. These cards were first introduced in the Yu-Gi-Oh! GX Anime in a forbidden "Underworld Deck", used exclusively by Zane Truesdale following his "rebirth".

However, there is a drawback to using this deck in the anime, hence it may be the reason why it is forbidden. The deck's unquenchable thirst for battle and improvement forces its owner go past his/her limit, slowly damaging the user's body over time. Because of this drawback, only duelists with strong wills and dreams of progression can use this deck for long periods of time. Depending on how strong willed the duelist is, the duelist using this deck will experience heart pain, which may lead to death, as displayed by Zane Truesdale after using this deck for so long. Even Syrus Truesdale experienced some heart pain when he decided to take over Zane's duel against Izimu Makoto, and use Zane's deck. This is the only known case of the drawback of this deck.

As a supporting theme, these cards can be splashable into many Dragon decks, given there are enough level 3 and lower dragons for support (Such as Masked Dragon, Armed Dragon LV3, Hunter Dragon, etc.). You can also build a deck based on those cards, by using some three copies of Hunter Dragon, some other dragons and cards that sends them to the graveyard. Apart from being 2000 - 2500 attack beatsticks, cyberdarks also works very well with all 3 virus cards - Eradicator Epidemic Virus, Deck Devastation Virus, and after the equipped dragon is destroyed, Crush Card Virus.