The Beast (Buffyverse)

The Beast was an apocalyptic demon that served as one of the "Big Bads" of Angel - his arrival began a storyarc in which Angel and his friends had to fight to save the entire city of L.A. from him, although powerful it was later revealed that The Beast was actually an enforcer of Jasmine - who would become another "Big Bad" in the Angel universe.

History
In 1789, the Beast was in Prussia, the Svear Priestesses were going to banish him from this dimension and had cast a spell so that he could not touch them. In an attempt to get assistance, he slaughtered several people in order to attract Angelus to him, at which point he attempted to create a mutual alliance with him. He wanted Angelus to kill the Svear and in return the Beast would pay him back somewhere down the line; Angelus refused and was subsequently beaten half to death before the priestesses arrived to banish the Beast.

In Apocalypse, Nowish, the Beast rose up from the pits of hell. The first people to encounter the Beast are Cordelia and Connor, who find the Beast arriving on Earth at the exact spot at which Connor had been born. A fight ensues, during which Cordelia is injured. Believing that he has something to do with the coming apocalypse, Connor distances himself from his teammates, who do not trust him. Angel, Gunn and Lorne decide to take on The Beast. Wesley rejoins them to help in the battle, but they fail nonetheless, and are all badly injured. The Beast then conjures the Rain of Fire over Los Angeles.

The Beast, seeking to reach Mesektet (The Little Girl of the White Room), kills everyone working at Wolfram & Hart as he tries to reach the White Room, turning the majority of his victims into zombies. The Beast kills Mesektet and drains her of her dark energy. Following the death of Mesektet, the Beast hunts down the rest of her "family", an order of mystical beings known as the Ra-Tet. Ma'at, Ashet and Semkhet are killed by the Beast. The final Ra-Tet, Manjet, is killed by Jasmine/Cordelia in secret. Using the metal wings in Ashet and Semkhet and the heart of Ma'at, the Beast makes a ritual to eclipse the sun, which is completed when the dark energy of Mesektet is imbued in the orb which was extracted from Manjet's head. The eclipse begins as a sunlight-blocking spot that spreads covering Los Angeles and is supposed to eventually cover the totality of earth.

Angel's team believe that their only hope is in Angel's evil alter ego Angelus, who apparently knew the Beast in the past, even though Angel himself does not remember the encounter. This is revealed to be because all references to the Beast in this dimension were magically erased shortly before its arrival; Angelus was the only entity in this dimension unaffected, as he did not technically exist at the time the spell was cast.

Once freed, Angelus was eventually convinced to reveal the details of his encounter with the Beast; the Beast had attempted to recruit Angelus in 1767 to stop some priestesses who were attempting to banish the Beast, but Angelus declined and was knocked out before the Beast was banished. After Angel's soul was stolen and a ritual to restore his soul via dark magic was faked, Angelus broke free and sought out the Beast. Wesley broke Faith out of jail to assist in Angelus' capture and fight the Beast, though the Slayer was little match for the physically imposing demon, who was in the end betrayed by Angelus, who stabbed him with the knife the Beast had carved out of his own bones as a tribute to his master. The death of the Beast also undid the solar eclipse, leaving Faith in the sunlight and Angelus confined to the shadows, much to the latter's annoyance (Angelus didn't expect that the Beast's death would restore the sun).