Beast's True Name is the Beast of 666 (666の獣, 666 no Jū?) that was written of in the Book of Revelation from the Bible. It is said to be the beast who wears ten crowns symbolizing the sins and greed of humanity. The entire purpose of the Holy Grail War is to bring it into existence, planned by a heretic cardinal who stole an imitation Holy Grail from the Church. It is to be a new being of a higher level of existence incarnated through the Greater Grail, a cauldron to give shape to it due to the shape of the Grail originating from Christianity, by using the souls of seven Servants as an ignition charge.
It is referred to as Beast VI.
Initially, the Beast is described to be a fleshy, black lumpy sea, due to having not fully materialized yet. But, as two of the crowns start appearing and it devours more sacrifices, it becomes able to generate arms out of the mud. After Manaka's death, the Beast develops a scarlet frame with 7 open mouths. It then becomes able to form heads prior to its first defeat. In it's full form, it is apparently a scarlet creature that is somewhat akin to a dragon. It is also capable of human speech.
Prior to its birth and during its formation process, the Beast has a child-like personality. It views Manaka Sajyou as its mother, and acts very much like child e.g. wanting to be praised by Manaka. The Beast also has a ferocious appetite and gets excited by emotions like fear. Its deepest desire is to be born into the world and it was deeply hurt and mortified when Manaka (it's mother) was killed, flying into a furious rage upon her death. During its rampage, the Beast acts in a manner akin to a ferocious predator.
Fate/Prototype: Fragments of Sky SilverEdit
In Fate/Prototype: Fragments of Sky Silver, the Beast Class Servant doesn't appear until the end of the novels.
Manaka manages to defeat all the other Servants in the First Holy Grail War and begins sacrificing various people to help manifest the Beast, somehow believing that the Beast will help bring about Saber's wish. The Beast then helps Manaka kill her father, Hiroki Sajyou, who was trying to save Ayaka. The Beast of 666 then becomes horrified when Saber kills Manaka and flies into a rampage upon her death, trying to kill both Saber and Ayaka, as well as hasten its own birth.
As Saber is about to disappear, he uses Excalibur Proto in conjunction with the borrowed powers of the other 6 Servants to destroy the Beast before it fully incarnates. However, since Saber was only able to break less than half of the 13 restraints, he was unable to fully defeat it, instead the blast places the Beast into a state of dormancy until the next Holy Grail War. During this time, the Beast revives Manaka in a zombified state, due to Saber tossing Manaka's body into the muck of the Greater Grail prior to its defeat.
After the defeat of Sancraid Phahn and the battle between Saber and Archer, Manaka is shown to be actively incarnating Beast along with the Archer, Lancer, Rider, Caster, Berserker, and Assassin class Servants from the previous war. Manaka still seeks to use Beast to fulfill Saber's wish.