He wasn't the oldest of the horrors, no that goes to the primordial Cain and Able of fear: The Man in Black and the Sack Man. Nor was he the most powerful, not when compared to great beasts such as the Leviathan and Cucuy. However, he was blessed with a vision that allowed him to see beyond the petty squabbling of the Netherworld.
He saw how the horrors were ignored or mocked by the supernatural world, treated as unworthy when compared to the likes of mages, dragons, and giants. He saw how the supernatural races used fear and intimidation--the horrors' bread and butter--to cow the mortal races. And he saw how the horrors wasted their potential on feuds between various factions over the scraps of childhood fears they had been so graciously given by the powers that be.
This frustrated the Bogeyman to no end. He tried to change the system, but all he managed to do was start another faction of horrors loyal to him, splintering the Netherworld even further. After a particularly nasty incursion by fey into Shadow over some minor issue that ultimately left hundreds of horrors dead, the Bogeyman had enough, and sought out the Prophet of Endings, the mad oracle of the horrors. The Prophet, it is said, fell to his knees, and called the Bogeyman "the Shadow and the Veil, the Eye and the Crown, the One Who Comes Before The Fall." The apocalyptic oracle gave the Bogeyman directions to the fey's base, a hidden citadel located at the literal heart of Shadow, the Pit from whence all horrors were spawned.
The Bogeyman was furious. The fey not only claiming part of the Netherworld as their own, but they were also hunting newly spawned horrors; little more than children. However, the Bogeyman realized he couldn't take this fortress on his own, and so, through politics and guile, convinced seven other factions to join him: the followers of the Man in Black, Grandmother Gryla, Cucuy, Bonhomme Sept-Heurs, Lagahoo, Oude Rode Ogen, and the Prophet.
Together, the factions were able to drive the fey from the citadel in a climatic battle with many losses on both sides. Once in possession of the citadel, the horrors realized it was not of fey manufacture, but instead something far older. It seemed to be from the age of angels, but its architecture was ominous and imposing, entirely unlike the celebratory styles of the heavenly host.
At the heart of the citadel was a great vaulted chamber, held up by carvings of the various races: there was mankind, shouldering the burdens of the world. There were giants, the shapers of land and sea. There were dragons, the lords of magic. There were jinn, the masters of wishes. There were reapers, the guardians of death. There were fey, the storytellers. There were spirits, the embodiments of everything. There were angels, the champions of hope. And there were horrors, the bringers of fear.
And in the middle of this room was a grand throne, carved of some unknown black stone. The Prophet of Endings said it was the throne of the "rightful ruler of the faded realm," and bade the Bogeyman to sit in it. There was discord among the other powerful horrors at this, but thankfully, the Man in Black--being the masterful politician he was--enthusiastically threw his support behind the Bogeyman. The Coffin Mage, and the Good Doctorfollowed suit, and then, begrudgingly, the Black Dog, the Queen of Beasts, and the Hungering Cold bent the knee as well.
Over the following centuries, the Bogeyman's control expanded across Shadow, bringing under his control dozens of other factions. He gave those who bent the knee willingly a place in his council, known as the Nightmare Court, though his left and right hands were ever the Man in Black and the Prophet of Endings. To further this show of unity, the Bogeyman gave the average horror a means to speak and address their concerns to the Court's members. Over the years, this became codified as the advisory body known as the Parliament of Shadows, an elected body that represented the desires of various groups of horrors to the King and Court. Indeed, the Parliament was so influential that the grand citadel itself became known as the Parliament... causing no end of confusion for later scholars.
The unity of the horrors made them a force to be reckoned with. Though alone, a horror could not stand against a demon or wizard, as a haunting they could fight, and win equal ground. It was this unity which he took to the Celestial Conclave one day in the early 9th Century AD, and convinced the Archangels to end the age of magic, and begin the age of science, the age of reason, the age of man.
The rest of this history is rather well-known. The angels and horrors fought a brutal war against the various other supernatural forces in the Middle East which ended in the supernatural races signing the Veil Treaty in Baghdad. The Bogeyman himself was at the signing, and famously proclaimed "Yehi láyla!" To translate from Hebrew, "Let there be night," or more symbolically, "Let there be adversity." The Veil spread across the Abbasid Caliphate, separating the mundane world from the magical. For the first time in humanity's history, they were masters of their own destiny.
And the horrors, masters of the shadows, were there to guide humanity, whispering to them and stoking their fears.
It's not known exactly when the Bogeyman vanished. He fades out of the records as the 9th Century progressed. References to his disappearance are first seen at the turn of the millennium, but they seem to indicate he'd been missing for some time by then. After he vanished, the Nightmare Court continued performing their duties as if nothing had changed, and when it became clear that the Bogeyman wasn't coming back, the Court declared itself the regent in his absence, while leadership of his particular faction was taken up by the once-human rogue known as Jack O'Lantern. Today, the Court still rules over the Netherworld, and the Parliament of Shadows still performs the messy duties of governing one of the oldest, and largest, nations in the world.
And, in the centre of the Chamber of Parliament, the black stone chair of the King of Shadows still sits, empty.