Great Evil Beast's History
The Great Evil Beast was formed when God uttered the words "Let There Be Light" and came to embody all the darkness in creation as well as everything that opposed the light of the Presence (which is DC universe's version of God) - however despite its immense power the Great Evil Beast was still a newborn entity and thus was easily defeated by the Presence and contained within its own realm.
When the events Crisis on Infinite Earths took place, a group of mystics named the Brujeria attempted an occult ritual designed to contact the Great Evil Beast but it went horribly wrong and the entity awoke as a furious beast, unable to grasp its power and becoming confused as to its existence.
The Great Evil Beast would encounter Etrigan, Dr. Fate and the Spectre - Etrigan taught the entity that Darkness was Evil, Dr. Fate told the entity Evil was Despisable and the Spectre finally told the entity about the concept of revenge: these new lessons angered the Great Evil Beast despite the heroes being well-meaning and it launched an assault on Heaven itself in order to get answers as to its existence from the Presence itself. Along the way the Great Evil Beast decimated anything that got in its way and was so powerful that Lucifer, Azrael and Beelzebub formed an alliance to defend themselves should the Great Evil Beast decide to attack Hell as well - somewhat ironically another batch of demons known as the Demons Three decided to do the opposite and allied with the Great Evil Beast, seeing it as the ultimate sin and evil (which technically it was). Finally, Swamp Thing, readied by his journeys into the Parliament Of Trees and his dealings with John Constantine, sought the entity out of his own free will, whereas all other beings had been engulfed after being soundly defeated. The entity listened as Swamp Thing gave what little he had, lessons that implied darkness was in its own way as needed as light, and had a place in the cosmos after all. It told Swamp Thing to leave him in peace, as he had entered, and prepared for what it called a great and final ending.
However as the Great Evil Beast stormed the gates of Heaven itself the entity extended its hand and much to the surprise of everyone the Presence did likewise - this climaxed in the two beings merging together into the perfect balance.
The battle was not without its price. Doctor Fate killed one of the Demons Three. On the more heroic side, Golden Age magical hero Zatara sacrificed his life to save his daughter Zatanna, and Sargon The Sorcerer also met his end. John Constantine lost some of his oldest friends in the campaign to stop the Brujeria, while his ally Mento (Steve Dayton, formerly of The Doom Patrol and an ally to The Teen Titans) glimpsed the smallest portion of the Great Evil Beast and went insane, actually becoming a villain for several years.
In later issues showing the afterlife in Swamp Thing, he would glimpse the joined hands of the darkness and the Presence.
This being is outside of the Presence's creation. This is no fallen Angel or Leviathan, this is the soul of darkness itself, a complete absence of divine light. This being is in every way equal to the power of the Presence, but his opposite who existed before Creation.
When the earth-shattering events of the Crisis on Infinite Earths struck, a group of mystic madmen calling themselves the Brujería attempted to conduct a mystic ritual to contact the Great Darkness; unsurprisingly, it went awry and instead the entity awoke as a furious beast, unknowing of its own power and confused about its existence.
It ended encountering a number of mystical heroes (Etrigan, Doctor Fate, the Spectre), who unwittingly angered the being and directed it against Heaven, leaving only the defeated heroes behind. They did this unknowingly, despite their best intentions; Etrigan taught it darkness was evil, Dr. Fate told him that evil was despisable, and the Spectre instructed it with the concept of revenge. Infuriated, the being sought to assault Heaven to demand answers from the Presence as to its existence, trampling all it crossed paths with. This power was so huge, it caused Hell's main three figures - Lucifer, Azazel and Beelzebub - to form a triumvirate to defend themselves should the Great Darkness decide it wanted Hell as its realm. A different faction, led by the Demons Three, banded together to follow the Great Darkness, whom they saw as the ultimate sin and evil.
However, before the Great Darkness reached the gates of the Silver City of Heaven, it encountered the Swamp Thing, who taught it the concept of good and evil, life and death, intertwined so that one could not survive without the other. The Great Darkness then extended its hand, and something happened that nobody had foreseen; the Presence extended His. When both Hands touched, both the ultimate light and the ultimate dark merged, forming the perfect balance as a form of Yin/Yang.
The realm of the Great Darkness, an expanse of complete and utter darkness known as the Sunless Sea or the Shadowlands, has been referred to as the power source for shadow-manipulating characters like Richard Swift (the Shade), his archenemy Culp, the Shadow Thief, Nightshade, Ian Karkull, and Alan Scott's son Obsidian.