Forbidden Beasts are bio-weapons that house a human body.
|Others||The Six Forbidden Magics|
|Body type||Humanoid (host)|
|Behind the Scenes|
|Universe||Guilty Gear Universe|
During the Crusades (the hundred year war between humans and gears), various organizations tried to create an anti-gear weapon derived from their technology. The end result was the six forbidden magics otherwise known as the forbidden beasts. The reason they are "forbidden" is that utilizing the magics resulted in extremely detrimental alterations of a subject, often making them no longer human. The Post-War Administration Bureau realized that making gears to combat more gears was a mistake, so they switched to using the six forbidden magics method. Once a beast is on the battlefield, gears instinctively know they are a natural enemy (in fact that's related directly to a gear's programming, strangely enough).
Forbidden Beasts can only be housed in humans after sacrificing something. In Zato-1's case, he sacrificed his eyesight to become a host to Eddie, a shadow forbidden beast. In Millia's case, she obtained Angra, the magical hair forbidden beast, when she was taken to the Assassin's Guild, though she was an unwilling participant. It is currently unknown what Millia had to sacrifice to have the beast on her hair. The third was an orphan from England, however, his body wasn't a suitable host for any of the beasts and therefore the subject, Venom, was rejected. Knowing all too well the aftereffects of the forbidden magics, Slayer secretly chose Millia to be the one to ultimately kill Zato-1 should the power prove too unstable for him to control.
They are sometimes referred to as "parasites", as they have to live in a human body just to stay alive, and also secretly kill their host, even when they're not aware. An example of this was Eddie, who, slowly but surely was eating away Zato's mind, until he lost control of his own body to the beast. This problem is present within all beasts, as Millia has worried she may one day lose control of herself to Angra, and even does in one (most likely non-canonical) ending in Guilty Gear XX Λ Core Plus. However, in one of Eddie's ending in Guilty Gear XX Λ Core Plus, just when Zato's body decays to the point where Eddie can't use it anymore and believes he will die, he doesn't. He then realizes that a forbidden beast's morality isn't the same as a human beings, and leaves Zato's body, now living on as it's own independent being. It's unknown if Forbidden Beasts can indeed survive without hosts, or this is simply a non-canonical "what if" type of ending.