The Beast is one of the main characters of OH GOD THE RAPTURE IS BURNING. He was formerly The Slender Man before being "condemned twice." He is a sort of tragic protagonist, a Fear who wants only what's best for the world but knows very little outside of being a monster.
In the Rapture Logs
In Act III
In Act III, it was a being of absolute unrecognizability, a creature that fluctuated between manifesting as an uncountable amount of nightmarish horrors and manifesting as an uncountable amount of nightmarish horrors all at once. It had been described as "nothing like the slender man" and "something beyond a Fear." The Beast resided in the Planck Level.
In Act IV
In the final act, The Beast follows The Harlot out of the Planck level and onto the Earth. No longer kept in a mass-defying oubliette, The Beast's molecules begin to reassemble, creating a creature that better resembles the slender man. A second "Beast" manifests at precisely the same instance in the universe, albeit far bigger, resembling a large solid hologram that follows it around. He also gains a scarf of blood and a hat of shadow.
The Beast spends the act roaming the Earth, gathering a massive army of Masked Massacrers, confronting the Fears, hunting Indisen, and searching once more for The Harlot who had escaped it. Around October 5th, The Beast has a conversation with Ten Speed (who at that moment took the form of The Catechist, Fear of the truth), learning that the world around him is nothing more than a work of fiction. The Beast misinterprets this, assuming that means there will undoubtedly be a bad ending, turning him suicidal.
He struggles with his sanity for a while, killing Phil Collins and then growing warmer to the protagonists. He witnesses the death of Tiresias, which cements the bad ending in his mind, electing to go to the Final Battle only out of obligation. He spends the whole finale trying to get away from the fighting to kill himself, but Rapture forces him to stay alive and to kill his allies. Ultimately, The Beast takes advantage of Rapture's emotional states of panic to kill himself, unnoticed before it's too late to stop him and preventing Rapture from coming in the process.
In The Cockroach Metamorphosis
The Beast has yet to appear in Rapture: The Cockroach Metamorphosis but has made an appearance in the 2013 Christmas Special, where it's revealed that he comes back to Earth once a year to deliver presents to everyone on Earth in an attempt to atone for his crimes.