The Weequay known as Hoole is one of the dark sides most terrible tools. Born circa 100 BBY, as far as can be concluded, Hoole was better known to the galaxy as Darth Atavus, a monster who slaughtered many Jedi at the Battle of Roon.
In 80 BBY the man who would become Darth Atavus was named Hoole, a Dark Jedi who managed to lure several Jedi who hunted him into a trap. Taking advantage of the criminal organisation he would soon rule, the Syndicate, he managed to kill most of his hunters and capture a young Jedi Apprentice named Xenly and few of Xenly´s allies. Xenly was the only Verpine Jedi known to this day. Something that had raised concern, for the Verpine were all connected into one hive-mind.
After several attempts by his arch-nemesis Asaja Morine and her apprentice Caleb Brandl to free Xenly, Hoole finally managed to escape with him. Hoole turned him, corrupted him with the guilt and anger about his failure and finally turned Xenly into his apprentice. He had a good reason to do so, for he knew about the concerns of the Jedi. And he thought the same. Connected to a whole species, Xenly could wield terrible power. Instead of one soldier at a time, Xenly could raise armies through his link to his people.
But things weren´t that easy. Helped by another prisoner of Hoole who had managed to escape, Siri Romar, the Verpine learned of Hoole´s plans. But only after Xenly had infiltrated Hive 44. Through his rage and power he had corrupted every single member of the hive, killing the Verpines and keeping the drones bended by his will. Breaking their small sense of individuality and made them a part of his dark will. Hoole used the powers of his apprentice. Xenly was easy to manipulate, as his sense of reality became more and more corrupted by the experience. He used Hive 44 - the Xenly, as they referred to themselves - in his many battles against Asaja Morine and her allies. Roche was finally saved, but Hoole only relocated to the Ithania-Fields. Asteroids drifting through a huge nebula.
Nevertheless, Hoole lacked control of the consequences of his ruthless experiment. The Verpine under his will didn´t develop normally, as Hoole forced them to multiply in unnatural speed and the Dark Side took hold of them, until they were eventually creatures without a will, driven by dark instincts. Every generation showed new mutations, until only an expert could recognize the Verpine in them. They were mere vessels of rage, contained only through the failing will of Xenly. Xenly himself, the Dark Overlord, was reduced to a catatonic body and a wandering mind with thousands of bodies. Hoole´s attempts to strike at his enemies became more and more desperate. He knew his time was running out. Eventually young Jedi Siri Romar ended Xenly´s suffering and killed him. Hoole was defeated, even if that last confrontation costed the Jedi gravely. The uncontrolled Xenly ripped his organisation apart as he had to flee the Nebula alone. Almost.
The Xenly were difficult to wipe out. Every one of them, like their original species - the Verpine, can reproduce without a partner. Hoole took only a single specimen of the corrupted beings and brought them to the Mestra Fields. He kept a low profile. Recovering from his wounds and the mental suffering he had felt for killing his nemisis - Asaja Morine. The Jedi eventually considered him dead. And in the field - guarded by allies from his time as a crimelord - the Xenly grew in numbers and power again. But they had no real conscience, no real intent for anything. For reasons Hoole did not understand they had not become vessels of the Dark Side but rather beasts bordering consciousness and filled with only few emotions. All of them dark and unending. The were like generators of the Dark Side, multiplying it for no reason, but the strengthening of darkness itself. Without Xenly himself and his will, his children were useless.
Shortly before the Battle of Yavin the Xenly were used on Eliad as some sort of biological weapon, wiping out the entire population of the planet. The Empire bombed the planet and put it under quarantine. The Xenly multiplied in darkness and silence within the quadrant 22/7 of the Mestra Asteroid field. Hoole went into deep hiding and hibernation to outlive the Emperor. He was never a servant. It is safe to assume that he travelled from time to time to gain knowledge of techniques he hoped would help him control the Xenly, and to keep his location a secret.
When Master Yoda died, the last Jedi knowing about their existence passed away.
Seeding the Xenly on Roon, Hoole sought to discover a way to control them without compromising the location of the hive. His experiments in creating a construct - the Sphere - to act as the original Xenly had were difficult, and by 30 ABY Hoole was getting nowhere. In the end he had to understand he held the ultimate weapon with no ability to use it. By the time he emerged from hiding again, the One Sith had discovered the Sphere, and its purpose, though they did not fully understand the Xenly.
Darth Wyyrlok II, the dark side genius that she was, figured out how to create a heart for the Sphere - placing a Sith meditation sphere within the void. It would take time, and Darth Krayt negotiated with Hoole his loyalty in exchange for the secrets to control the Sphere, and thus the Xenly. Hoole became Darth Atavus, working within the Sith Court to maintain his project, and to recover. Krayt made efforts to use Atavus to kill Darth Insipid, but to no avail - eventually the two, while working with Yu'Dor, formed a triumvirate of sorts, acknowledging their respective individual aims.
By the time the Sith-Imperial War broke out, the Sphere was complete, and Atavus was ready to slaughter as many Jedi as he could. TO BE COMPLETED
TO BE COMPLETED