The Minus Curse is a spell that causes unusual backwards phenomena to occur in the Duel Monsters Spirit World (or at least the Ancient Forest). The Minus Curse is triggered by Cursed Needles.
The curse was being influenced by the Zeman the Ape King who currently rules the spirit's world and later his master Devack.
When the Cursed Needle first appeared, spirits were taken captive or turned into children. The captivated spirits were turned into stone tablets, resembling Duel Monsters cards, but with their Levels reversed into Negative Levels. Eventually, those spirits where used as sacrifices to power the summoning of Devack's god, Cusillu.
Ancient Fairy Dragon was among those captivated, but wasn't fully transformed into a "minus".
After Devack's defeat at the hands of both Leo and Luna, the curse was broken, releasing all the captive spirits and reviving the ones sacrificed to the Earthbound Immortal.