The Beast was a creature in the Garden of Eden that refused to be named by Adam and actively wanted to destroy humanity, and is the titular Beast described in the Book of Revelations. Because it was unnamed, it was shapeless and without physical form, but has nigh-omnipotent power to possess entire civilizations. It was kept at bay by The Shadow Dog, which would rise up whenever it attempted to pass through the doors between this world and the next. As a result, people incorrectly began to think that the Dog was the cause of the problems rather than its cure. When the Shadow Dog began to surface again in the late 2000s John Constantine created a task force to kill it, inadvertently releasing The Beast. The Beast then took over the minds of almost all humanity, causing mass rioting, murder, rape and suicide the world over, and even defeating Constantine's team and Swamp Thing. Eventually a half-dead Constantine, his then-girlfriend Angie Spatchcock, cab driver Chas Chandler, young magician Gemma Masters and plant elemental Swamp Thing were able to resurrect the Shadow Dog, destroying The Beast once and for all.
As one of John Constantine's most powerful enemies, the Beast has nigh-omnipotent control over every subconscious mind of the mortal realm including those of superhumans (i.e. Timothy Hunter). He has the powers to control the consciousness of the world, and sadistically uses them to commit acts of immorality, gruesome murder and suicide. The only ones that were hindering his powers were the Shadow Dog and John's team of magicians. The Beast is also strong enough to rip the soul off even the toughest superhumans, most notably Swamp Thing.
Having control of the subconscious, the Beast is invulnerable to every harm, and can only be destroyed by the Shadow Dog whom God has specifically created to combat him.