The Judge was an ancient and legendary demon from the Middle Ages, who was brought forth to rid Earth of the plague of humanity. He was aptly named The Judge for he had come to separate the righteous from the wicked, and burn the righteous down. An army was sent against the Judge, and though most of the warriors died, the army was finally able to dismember the Judge, but not to kill him. The pieces were placed in iron boxes and scattered to be buried "in every corner of the Earth". For six hundred years he remained aware of his status.
As Drusilla's 1998 birthday present, Spike has all the pieces of the Judge brought to Sunnydale. Drusilla plans to reassemble the Judge and unleash Armageddon. Spike's minions succeed in reuniting all the pieces and taking them to Spike and Dru's lair, except for one of the arms, which Dalton loses to Buffy. At The Bronze, Buffy opens the box, and the Judge's arm immediately attacks her.
Fearing that the Judge would be too dangerous to fight, Angel decides to take a ship to hide the arm in Nepal, thus leaving the Judge incomplete, which would have ended Dru's plot to reawaken the demon. However, Dalton and Spike's minions manage to steal the arm back from Buffy and Angel.
With the pieces complete, the Judge is reassembled before an ecstatic Drusilla. The Judge shows contempt for her and Spike as the two share affection, a quality considered by the Judge to be human. The Judge wanted to kill them until Spike reminded him that he and Dru had brought him back. Deeming them "helpful" for his purposes the Judge agreed to leave the two unharmed. Seconds later, the Judge turns on Dalton, whom the Judge also deems "full of feeling" (Dalton's love for knowledge). As he had just been awakened, the Judge wasn't at his full power, and needed to touch his victims to incinerate them, as he demonstrates on Dalton, much to Drusilla's delight.
Buffy and Angel find themselves in mortal danger when they go to the Factory, only to find the Judge already assembled, and Spike and Dru ready for their visit thanks to Drusilla's premonitions. Buffy and Angel barely escape, Buffy having already felt the Judge's deadly power when she kicked the demon. The two hide in Angel's apartment, where they share an intimate encounter, which causes Angel to lose his soul. Meanwhile, the Judge rests, waiting for his powers to return to full strength.
The now unsouled Angel returns to the Factory. The Judge attacks him but is unable to incinerate him, as Angelus is "clean of humanity", according to the Judge. While it has been established that all vampires (see Vampire) have some humanity in them, the Judge's powers do not work on Angelus, despite the fact that he's a vampire, as he's incapable of feeling love or affection (unlike Spike and Dru or Dalton), and is a true creature of evil, meaning that he is capable of surviving the touch of the Judge.
With his powers restored, the Judge is taken by Angelus and Dru for a massacre at a mall. With a mere gesture, the Judge incinerates a man and then attacks a large number of people. The Judge's power bounces from a human to the next, thus creating a web of victims. Before the victims die, Buffy attacks the Judge with a crossbow. The Judge reminds Buffy that no weapon forged can hurt him, prompting Buffy to fire a rocket launcher at him. Unfamiliar with modern weaponry, the Judge merely stands as the projectile flies towards him. Angel and Dru escape, while the Judge is destroyed, though not killed, by the explosion. The pieces of the Judge are then picked by the Scoobies, who proceed to dispose of them, taking great care to keep them separate.
It is likely that the Scooby Gang buried The Judge around Sunnydale, in which case he was most likely swallowed into the Hellmouth in the season seven episode "Chosen" when all of Sunnydale collapsed.
The Judge is a human-like tall and lumbering demon with blue skin and solidly black eyes. His body is covered by a metallic armor and his head sports three bone crests.
The Judge's main power was to "burn the humanity" out of a person. The "humanity" in question is not the humanity in the personality of the victim, as the personality of a vampire is a result of the human predecessor, but the emotional capacity of the Judge's target. Angelus is immune to the Judge because he is a truly evil being, incapable of affection or love, unlike Spike and Drusilla.
The Judge was also capable of sensing the emotions and feelings of others, as proven by the fact that he instantly senses that Spike and Dru are lovers and that Dalton has a love for reading and knowledge.