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).
Powers And Abilities
The Beast was an extremely powerful Demon that possessed Superhuman strength, stamina, durability, resistance and near-invulnerability far superior to most if not all vampires, demons and slayers (at least those that interacted with the earth) - capable of effortlessly defeating the entire Angel Investigations team during their first battle, even going so far as to stab Angel in the neck with his own stake. He also possessed a rock-like hide, rendering him extremely resistant to physical damage; he withstood attacks from Angel and Faith without any sign of damage, and even shotgun blasts did little more than knock him off-balance for a few seconds. His strength allowed him to perform massive leaps. The Beast also hit Connor and Faith on separate occasions with a reverse punch which sent both flying back approximately ten to fifteen feet with no apparent downward arc in their trajectory, and in both cases their momentum was only stopped by impacting a large, solid object (in Connor's case, a metal dumpster, and in Faith's case, a large storage crate which was shattered by the force that she impacted it with).
Furthermore, the Beast appeared to be capable of teleporting between dimensions; during one battle with Angel Investigations, he was banished to another dimension by a spell cast by Wesley and Fred, only to reappear behind them unscathed as soon as the portal closed.
However, the Beast had two weaknesses. The most obvious weakness is that his bulk made him incredibly slow, leaving him outmatched by Angelus' speed, allowing the vampire to easily avoid his blows and even find time to crack jokes while doing so (however, it should be noted that Angelus was more concerned with dodging the Beast's attacks than actually fighting back). Contrary to this, the Beast was seen quickly disappearing from frame after he killed Ashet, showing speed similar to vampires, but this may have been an extension of his dimensional teleportation ability rather than actual speed. In addition, the material of which his own bones were composed of was capable of piercing through his skin, as shown when Angelus killed the Beast with the dagger made from its own bones. However, the difficulty involved in acquiring such a weapon make it hard to exploit this disadvantage.
In addition to his almost limitless physical abilities, the Beast had great magical abilities, allowing him to perform spells and rituals such as the Rain of Fire, draining Mesektet of her dark essence and performing the ritual to blot out the sun, although it is unclear whether he acquired this power himself or was simply using power that his 'master' had provided him with.
He may also have great healing abilities as he was able to carve out part of his own bones to create a dagger for Jasmine and yet was unaffected.