Forbidden Magic is every spell ever crafted that the Maguc Council have deemed to powerful to be permitted. The incantations are transcribed by mind wiped slaves under the scruitiny of the Lord Darkside and then locked in the great Vault under the Research Sect Headquarters. The vault is protected by some of the most powerful defensive Wards in Vorn City, even stronger then those that protect the Magus Council themselves, and for good reason.
The spells contained within the Vault could destroy the entire planet. Spells like Ragnorok which change every living thing into a destructive beserker, Apocalypse which destroys every Mani on Tal'Vorn, Divine Word which, when spoken, kills everything within ear-shot, the caster included. Such spells were crafted in the time after Magic was discovered, the newly formed Research Sect pushing the bounds of what could be done further and furhter back until they finally reached levels too dangerous to even cast under cointrolled circumstances.
And yet not all of these spells are totally banned. Each Sect has access to just two Forbidden Spells that were granted to them by the Magus Council, against the wishes of both the Research Sect and the Darksiders. Unfortunatly this occured before the Dark Sect was riven from the rest of the Council and so they too have access to a pair of Forbidden Spells, the infamous Dark Bolt which dissolves the flesh of living creatures and cannot be healed or regenerated leaving amputation the only option and the equally brutal Puppet Master which allows the Dark Mage to control the actions of another. These spells are just a taster of the horror that would occur should the Vault ever be breached . . .