The Old Ones

"In elder days there were other things; Old Ones, older than the gods, from a place where things aren't like they are here. They came from the stars, and in the depths of the primordial seas they took their shapes, and held dominion over the earth during it's infant eons. The Old Ones were here when the ocean was young, and when the first things wriggled and writhed on the oozing shores of the primal world. They were here when the first men awoke, and for unfathomed ages they enslaved the children of earth with their thoughts. Thus was the age of the great Aboleth, until the time of the gods, who smote down the Old Ones, and buried them with their cities beneath the waves."

"...But the Old Ones never die, and they never forget. They remember the days of their sovereignty, for the memory of the Aboleth is eternal, and their plans for return span the epochs of time. From black depths beneath sunken ruins they pull the strings of history, speaking to the world of men in dreams, and transmuted thought. Thus in their tombs they wait, until all the pieces are in place; for when the time is right, the Old Ones will rise again… and on that dark day, a reign of madness shall return to a godless world."

-Ulo the Historian

Overview
The mythological Old Ones were an ancient and malevolent species of elder gods or quasi-deities that once ruled over the Prime Material plane, enslaving ancient humanity. Eventually the gods arrived and ended the reign of the Old Ones, who fled or were buried in the world's oceans.

References in Mythologies & Religion
Most, if not all, cultures and religions reference, in some way, an ancient age of enslavement that mankind was broken free from.

Spaurian mythology claims that history began after an "age of madness" lasting one thousand years, where humanity was enslaved under nameless quasi-deities, before their empire was smote down by the arrival of the gods. The Old Ones, as they were called, were either slain or robbed of their powers, and cast into the ocean, buried within their ancient cities that sunk beneath the waves.

Throughout the ages, obscure cults of dark sorcery in Nurn have called out to "ancient evils of the sea." Such cults have risen in numbers and intensity within the last hundred years.

Idalidarian Elves speak of a similar-sounding age of darkness that overshadowed humanity before their expeditions into the Prime Material world.

Return
Ulo the Historian claims that the OId Ones still live in the dark depths of the sea, manipulating the world above, and "putting all the pieces in place" for their return to a world without gods. It is implied that the Old Ones speak in a form of transmuted thought, suggested to be dreams or visions. Though they are mentioned very few times in Ulo's works, the Old Ones are to be the cause of the ultimate doomsday; one that will once again enslave humanity with eternal madness.

The Bois Campaign
Around the beginning of the Bois campaign, Captain Draz traded information with Derek Merek to obtain the location of a sunken, black Monolith that Merek had discovered in his prime. During their underwater exploration to find the Tablet, Draz, Nathan Janson and Quinn Monroe found the Monolith, and accidentally awoke the slumbering Aboleth within, resulting in the entire megastructure rising above the waves and releasing it's ooze.

During their time at sea, the Bois encountered the worst of the Aboleth's influence, as the long-dormant god began psychically reaching out to human beings in the port towns surrounding it's tomb, while ooze from the Monolith gathered on the shorelines. After their first meeting with Qrow, the Bois discovered that Skullmarsh and two other coastal towns had been drained of humans, who had been gathered to the Monolith to power the Unsealing ritual with the help of William Wade, the Dark Eyed Fellow. The patterns followed by the multitudes of the enslaved would be the "key" to opening the seal.

With the help of Midas the Mage and the sacrifice of Cadence Qrow, the Bois were able to defeat the Aboleth in it's physical form even after the ritual had released it back into the seas. This accomplishment would later be described by Midas as "a critical piece of the puzzle" that would save and preserve future generations.

The Aboleth
The Aboleth itself was a physical manifestation of an ancient Old One, resembling a gigantic catfish-like aberration with multiple tentacles and three vertically ascending eyes. Being a predominantly psychic being, the form encountered by the Bois likely was not it's original. It's psychicly-powered healing during battle and rapid degeneration after death suggests that it was not birthed naturally, but rather, constructed by the alien mind from the ooze and transformative matter contained within the deeps of the Monolith itself.

The Aboleth possessed unparalleled physical strength, and was the ultimate undersea organism in terms of maneuverability and deadliness, rivaled only by Mackintosh's dragon turtle form. It's intelligence was likely beyond that of any known being, judging by it's vastly powerful psychic abilities and telepathy. Using it's mind, the Aboleth was able to precisely manipulate a vast number of human beings with ease, and even drain psychic energy from them to repair it's damaged body. It also possessed a psychic "shockwave" ability that allowed it to attack the minds of creatures nearby, using water as a conduit.

Trivia

 * Case Name "Skullmarsh Incidents" : Location: Off-Land : ENC, CNJ, ??? : Report to CROWNSMAGE :
 * The Herald of the Aboleth was an unfortunate human wizard named William Wade, who, a decade earlier, was exposed to the otherworldly horror of the Old Ones while exploring a frozen megastructure with his voyager crew. Cadence Qrow/Rose glimpsed a fleeting vision of the horror in the mountains when he inflicted the Phantasmal Killer spell, conjuring Wade's deepest fear. In the end, the maddened Herald was consumed by the Aboleth as it rose.