Nihhilus wasn't always the powerful Sith he is now. He was born into slavery, but his slaver wasn't a kind one, the slaver ended up being a Powerful Sith himself and Nihhilus being around watching him, was learning even though his Slaver didn't intend for Nih to learn his techniques. But Nih did, he watched and respected his Slaver. At the age of 15, Nihhilus was strong in the Force, training while his Slaver was away or sleeping. Nihhilus never stopped he was fascinated with what he was learning. But he wasn't the only slave, nor the favorite. So he came up with an Idea for his Slaver to notice him, Kill the other Slaves. So while the other slaves were on errands, Nihhilus assassinated each and every single one of them, only took him 1 week to for fill his duties. As the Slaver started noticing his slaves were disappearing, he started paying attention.
But as all the Slaves were gone the Sith found out who it was, Nih. The Slaver/Sith was not happy, with his power, Nihhilus was cast against the wall and shocked with lighting for a while, Not Dying, the slaver was interested in how far he could push Nihhilus's Strength. So after nearing Nihhilus to his death, he drug him outside and told him to try his best to hurt the Slaver. Nihhilus did as he was asked.. Not knowing of how to control his powers he killed the Slaver with a flick of his wrist, Nihhilus was free. But he knew he would be hunted if anyone found out. So he buried the body and went on with his Destiny.
He began rampaging through his Slavers items and storage and came across a datapad, on the Datapad was the coordinates of a Planet where they trained people like him, interested in what it was, he headed towards the planet with his Slavers ship. As he came upon Korriban he didn't expect what he had to go through to gain the title of "Acolyte". But he succeeded his training, and became the Apprentice of a Darth Zash. She sent him to a Planet Dromund Kaas, there he didn't know anyone or anything, but he pushed his way through the planet as any other Apprentice did, until he came acrossed a Sith that didn't have the Sith Attributes he was supposed to have, the Sith's name was Ja'cole. They never abandoned one any other as their friendship grew to trust then to Brotherhood. There on Dromund Kaas they decided to become stronger so they designed a Guild, with two others known as (Unknown), and Volcanous.
They called this Guild, "The League Of Assassins". They swore to keep the Guild based on Revan's beliefs, story, past, and his Lore. That didn't last long, they weren't getting the Members they needed to build so they changed their ways and became a free Guild using Respect, they Voted Nihhilus Leader, and Ja'cole became his trusted Ally, Friend, and Brother. He was the second in command.
The Guild Grew, and grew, and grew. Until what it has become today. Ja'cole stepped down and Nihhilus needed another Friend, he came across and interesting Sith named Valizar. Nihhilus gave him the chance to become his Friend. Valizar has never disappointed him.
Nihhilus to this day is still Leader of the League of Assassins. Showing that he can hold a Brotherhood together and never let it slip.