The Legendary Forbidden Magic is a set of dangerously powerful spells of Dragon Quest: The Adventure of Dai.

Their most significant drawback is that these spells reduce the lifespan of their caster, and as such they are considered outside of what mages should dabble with, especially human ones, as monsters and Darklings have a slighty higher resistance to its side-effects, although not by much.

Known such spells include

• In ice, the legendary spell used by Flazzard to incase Leona, which puts someone in an unmeltable and unbreakable ice coffin, making this prison indestructible for as long as the caster of the spell still lives; furthermore, the ice quickly saps away the life-force of its victim, killing them in the span of around two days. Once the caster is dead, the imprisoned victim could theoretically break free of the ice by themselves, but should their life-force be too weakened by that point, they risk of not having the strength to do so. And even once the spellcaster is dead, the ice is still very difficult to melt, being completely unaffected by anything below or equal in power of the average Frizzle; Leona's ice prison, after being hit by multiple Frizzle spells from Popp unscated, took a combined Frizz and Frizzle, stored contemporarily in the same Magic Bullet of Maam's Magic Bullet Gun, before it finally melted away.
• The Blizzblaze Curse Field, a special spell casted by Flazzard with the cooperation of his Blizzblaze Legion. Amplifying the power of Flazzard's own core made of opposite energies thanks to the use of two Dark Spires made of said elements, it results in an energy field that severely weakens all the people opposing the caster of the spell present within the field, reducing their physical and magical abilities by five times.
• The unnamed legendary spell used by Hadlar to create monsters like Grandard, Flazzard, and his five Orichalcum Guards; King Vearn himself used the same spell to give life to the other eleven chess pieces, although only Maximum, the king piece, was given personality. According to Matoriv, "as beings created from forbidden magic, they take traits from the soul of their creator, reflecting them in their individual personalities", meaning that the monsters that Hadlar and Vearn created from forbidden magic had some of their traits and abilities. However, because the creations are not technically living, healing spells, and by extension the Refractor Fist, do not work on them, requiring instead, depending on their body structure, their creator's direct assistance in order to regenerate injuries. Furthermore, as the created monsters are fundamentally extensions of their creator, should the latter perish, the beings made with this forbidden magic spell will also cease to live immediately; however, Hym was able to avoid such fate only due the miracle inheritance of Hadlar's soul, becoming by all means a truly living being, marked by him also obtaining the ability to be healed by healing magic. All beings created by this forbidden spell have been shown to possess a core, which is the center of their being; should it being damaged or destroyed, the created monster will significantly be crippled, or straight up die in some cases.
  • An unnamed, similar, perhaps the same, spell was seen in the prequel manga, being used by Hadlar to create the undead knight Bartos, by fusing the corpse remains of six great swordsmen the Dark Lord had freshly slain on the battlefield, creating a monster that combined all of their sword fighting skills into a single being. Unlike all other early creations of Hadlar however, Bartos was characterized by a noble, righteous and kind heart, meaning that it may be not the same spell after all.
• The Five Finger Flares is a Coup de Grâce close to Forbidden Magic, which allows the caster to cast five Kafrizz spells at once.

Trivia

• It is unclear whether Legolu, a spell animating rocks shown in the second film, is intended to be the legendary spell used by Hadlar to create Flazzard or not.