A/N: I'm doing something I never thought I'd do; I'm posting an actual story! I wrote this for my daughter, who begged me to write it down, and for my son, who said he would like to read a story like that. And for myself, just to see if I could.

The original plot germ for Krayt's Oath owes a debt of inspiration to RoxanneRay's story "Recovery." I read the first three chapters as she published them several years ago and was intrigued by the premise. Although this tale takes quite a different tone and tack than hers, her concept provided the original spark for my own ideas.

Many thanks to my faithful betas: DinoDigger, QueenSqueaker, CLM, and my daughter RecklessRonto, who also drew the fantastic cover art.

A note on canon: Episodes I through VI; with lore to suit my purposes drawn from both the EU and Disney (though I have taken artistic license with some details) and a bit of my own world building. AU from the end of ROTS.

N.B. I made up my own astronomy and medical science. Please ignore any inaccuracies, irregularities, or downright impossibilities.

Dedication: To my daughter for encouragement, brainstorming, and feedback; my son for asking every time I sat down at my computer, "Mom, are you going to write?" and my husband who, though only a casual SW fan, nevertheless cheered on my new obsession. And to the C and Y families for reasons they will understand.

Prologue: Dies Irae

Darth Vader simmered with irritation. For weeks the holocron in front of him had resisted his efforts to open it. He had surreptitiously accessed his master's private library in the Imperial Palace, certain that Sidious was keeping secrets from him, just as the Jedi Council had. It did not matter in the end. Jedi or Sith, they were all the same. They wanted his power, but only if they could control it. Both kept the deepest secrets of their Orders and the Force from him. But no more. He would not be held back any longer. He was determined to plumb the depths of this particular holocron, which had belonged to Sidious's master, Darth Plagueis. Perhaps at last (too late, a small voice whispered) he would learn the secret for which he had sold his soul.

With grim resolution, he stoked the fires of the Dark Side until it was a raging inferno within him. Patiently, he bottled it up as Sidious had taught him, concentrating it and honing it until it was a razor-sharp weapon to be directed as he chose. The exacting discipline was exhausting, but at last he leveled the roiling energy at the holocron. It resisted his demand. He persisted, determined to prevail. The struggle continued for an indeterminate period until it submitted to his will at last.

"Who summons me?" the hologram of Darth Plagueis asked.

"Darth Vader." His voice boomed through the vocoder.

"Lord Vader, you have proven worthy to open the holocron. What knowledge do you seek?"

"To learn the secrets of the Sith Order—specifically the method for keeping someone alive through manipulating the midichlorians."

Darth Plagueis's cadaverous image straightened its back and assumed a professorial demeanor. "The Sith Order has long sought a procedure to manipulate the midichlorians to create life spontaneously. This technique has not yet been discovered."

"My master, Darth Sidious, claimed that you had discovered it."

Darth Plagueis paused as the program searched its memory files. "If so, that knowledge has not been recorded within this databank. However, there is a related technique to preserve a dying life. It is time-consuming, complex, and difficult, and it requires an uninhibited connection to the Dark Side. The procedure begins with creating—or in some cases, identifying—a link in the Force between two individuals: the one who is near death and another who is healthy. Through this connection, the Dark Lord can siphon the life energy of the healthy individual into the one who is dying. During the course of the operation, he must concentrate fully upon the process. The least distraction or interruption will result in its failure. However, provided that he is able to maintain the flow of energies between the two persons, the one who was dying will survive. Do you wish to study this technique?"

Vader had listened in fascination at first, but by the end of this speech he was filled with dread. His gut twisted with anxiety and, though he would not acknowledge it, guilt. Darth Plagueis ceased speaking and returned to his neutral posture. It took Vader several long minutes to collect himself enough to say slowly, "Lord Plagueis, what happens to the healthy individual as a result of this operation?"

"He dies, of course, Lord Vader. There is always a price. As a Dark Lord of the Sith you already know that all power comes at a cost. The greater the power, the greater the price that must be paid." The holocron fell silent again, and Vader lowered his head. For a long time the only sound in the small room was the rhythmic hiss of his respirator.

So. It had been his fault, after all. He had killed her, in a manner of speaking. It was her life that had sustained his. Grief and guilt threatened to consume him. He wanted to shriek his fury to the stars. Destroy the palace. Even the entire district. Slaughter a legion in a battle frenzy. Choke a hundred incompetent idiots to death.

But his fury was impotent. Distant. Undisciplined and undirected. This was not fury that would aid him in the Dark Side. It was a mass of guilt and regret and heart-wrenching sorrow. He could not summon the discipline necessary to harness it, to shape it into a weapon to wield against his master. Not now that he stared at the devastation of his life. Now that all the masks had been stripped away. Now that he could see clearly exactly how skillfully his master had manipulated him and his fears.

Only the power of the Dark Side can save her, Sidious had said. But he had never intended to use it to save her. Her only value to him had been to bring Vader to his knees before a new master. Once that was accomplished, she was only a liability to him. The truth was ashes in Vader's mouth.

He never knew how long he sat in that darkened room, mourning the ruin of his life. The fall of his last idol. The man he had respected and trusted above all others, who had offered him hope when he was desperate, who had been the only one not to betray him on that last terrible day. Who, in the end, had betrayed him more thoroughly than Kenobi or his wife ever had. It was a bitter truth to swallow—that not even Palpatine had truly valued him. That not one person except his mother had ever wanted him just for himself.

When he rose at last, it was with the bleak determination never to be used again. He could not destroy Sidious. He lacked the necessary resources and had not established a power base. He had not believed he was ready to take over the Empire. Had known that he needed more training. So he had not bothered even to lay the groundwork yet. He supposed he could begin now. Form plans that would come to fruition a decade or two from now. But it seemed like such a waste of effort. He had never particularly cared about the Empire. Or the Republic. He had wanted to gain power so he could save the people he cared about. So he could govern his own life. Well, power had been an illusion. And all the people he cared about were dead. Most of them had betrayed him too. But he could direct his own life. From this day forward, he would call no man master. He would please himself and only himself. The galaxy could spin off into the abyss.

He swept the holocron closed. "Thank you, Lord Plagueis."