In order to introduce the readers more quickly to this AU, there will be a single non-canon fact that is related to the chapter in the header of every chapter. Here is the first one:
The Celestial War (or the Celestial Civil War) was a war fought before 35,000 BBY between the Imperialist and the Anarchist factions of the Celestial civilization. It ended with an Anarchist victory and was shortly followed by a war with the Rakata, which sealed the fate of the ancient civilization.
22 BBY, Mortis
It was him.
After what he had done thousands of years ago, he showed no sign of regret or remorse. The Father was angered by his lack of emotion, his apathetic attitude. It was as if he didn't care for the trillions of lives lost because of him, for the balance he disrupted that still had not been restored. As the antithesis of his very persona stood before him, he knew that this would be one of his most important trials. Still, he had no intention to let the Slayer have his way. He threw his anger away and calmed himself. He tried, in vain, to find a weakness in the man before him, to formulate a strategy that would grant him victory.
However, his children could not display such cold-bloodedness.
"How dare you come to Mortis, you monster!" The Daughter's anger was evident from the tone of her voice. The skies of Mortis darkened, as her rage reverberated throughout the planet.
The Slayer laughed. "Monster? You, who care not for the lives of the people whose wills you represent, call me a monster? It seems that the Celestials have retained a sense of humor over the years."
"How dare you speak of them, of us, after what you have done? Your very existence is an insult to the Force, and I will end it right here!"
The Daughter fired an orb of pure Force energy at the Slayer, with the explosion from the impact pulverizing a quarter of the Monastery to an extent that there was not the smallest particle of it left. For a moment, the Daughter thought that the fight had been won.
"Was that supposed to be an attack? You insult your ancestors, Celestial."
The Daughter turned around, and, in horror, saw the Slayer standing behind her without a single scratch on him.
"How-"
"Do you really think you are worthy of being the representative of the light side? Your ignorance is beyond curing; and your lack of Force prowess is an insult to your ancestors and your Father alike. You are boring - perhaps the Son will entertain me more."
The Son had been watching, but he had not said a single word. Unlike his sister, he had no regard for the lives that the Slayer took some millennia ago. He knew that he couldn't even dream of defeating the Slayer. However, that didn't mean that he couldn't join him and leave this excuse for a planet. Although he loved his sister, he loved himself more.
"I have no intention to fight you. I do not care about what you did back then. Let me become your apprentice - and teach me about the true secrets of the Force, those that my Father denied me." The Son felt frustrated over having to make such an offer, but the power of the Slayer was simply too great.
"Oh? So you would throw away your pride in your pursuit of greater power? An action worthy of a master of the Dark Side. However, power is worthless if it is not used. What do you intend to accomplish if you evolve past what you are now?"
The Son was surprised by this question. The users of the dark side, regardless of who they were, always strived for superiority and destruction. Surely the Slayer, a true master of the dark side, knew this.
"I would destroy all that I could, and leave nothing behind."
The Slayer smiled. "I presume you have no plans for after the destruction?"
"Plans? Destruction itself is my goal - I seek nothing else."
The Slayer looked at the Son like an adult would look at a 3-year old child. "I thought you had potential when you offered to be my apprentice, but in the end it seems like you are just another star in the sky. If you truly believe the dark side is about destruction, you are easily more ignorant than your sister."
The Son, like the Daughter, was shocked. Not at the Slayer's unbelievable power, but at his strange philosophy that he could not comprehend. He had never been held in contempt like he was now, not even by his Father. There was no mistaking that the Slayer was a master of the dark side - he could feel the intense dark aura around him. So what was the reason for his expression, for his disappointment?
"Why do you think my answer is unworthy?"
The Slayer pondered for a second whether he should take the time to explain this to a dead man, but he saw no reason not to. "The light and dark sides of the Force do not represent good and evil. They represent conservation and dynamism. For a follower of the dark side, if there is no change in the universe, that universe is worthless. And yet, the universe you envision is the very definition of stagnation. To a true user of the dark side, your dream would be a nightmare. You are unworthy of your title, like your sister. If you had proven to be worthy, I would have spared your life; but as you are now, you are nothing more than an obstacle."
The Son and the Daughter saw the Slayer draw his blade slightly and then sheathe it, accompanied by a flash of blue light.
"As such, I shall have you die here."
It took a moment for the Son and the Daughter to realize that both of their hearts had been pierced by the Slayer in an instant. As they realized, so did they vanish from the universe in an explosion of Force energy.
The Father watched in horror. He never thought such a technique could be used by a human so easily - perhaps he had been in this prison for too long, distant from the reality of the universe. In his weak state, he didn't stand a chance against the legendary assassin of the Celestial War. Although plagued by grief, the Father knew what he had to do.
In a single moment, the Father grabbed the Mortis Dagger and stabbed himself in the heart. Impressed by this act of heroism, the Slayer of the Celestials looked on with respect as the Father's being faded from the universe. As the Father disappeared completely and Mortis started to fall apart, the Slayer mused to himself:
"It looks like things are about to start moving."
The first few chapters can be confusing because of the OCs introduced, but I do not want to deny the reader the pleasure of figuring out certain elements of the story by over-explaining things. A hint: the Origins aren't a new species, they already exist in the Star Wars universe.
For the record, the Celestials are a canon species who were overthrown by the Rakatans in the Celestial-Rakatan War in approximately 35,000 BBY. It is heavily implied in the lore that the Ones have a connection to the Celestials - this connection is solidifed in the story. More about their connection, as well as the Celestial-Rakatan War, will be revealed in later chapters (and maybe another fic set in the same universe).