How does a simple soldier become a king? 

By betraying an empire for the sake of others. 

Of course, if one were to ask Saber about the events that led to his being crowned king, the man would simply state that it was never done for any throne or recognition. Rather, that he betrayed Rome because it was the right thing to do. Not that many would believe such a humble admission, however. Certainly not one that comes from a man with cold, empty blue eyes that look like chips of ice. 

To boys born in Imperial Rome the future came along on only a few accepted paths: the legion, the senate, mercantile life, art, or priesthood. For Ambrosius Aurelianus, a quiet and pale boy of a smaller than usual stature, it seemed all but certain he would end up in a temple. Or, if he was lucky, to be selected by the Emperor's son of similar age to himself to be a part of his personal servants. Neither option truly appealed to the boy as he grew and at the age of fourteen, when it was time for him to begin servant lessons or to be selected to a temple instead he turned away. 

The legion was his choice for he chased after a star on the horizon, one he knew he'd never be able to reach if he didn't have some way to chase that distant light. 

Eventually his path took him to Britain, to a small fort that was not even a proper castrum. Instead a border outpost in the western part of the land, looking over the growing threat of a kingdom that had come into being. A kingdom ruled by the newly crowned King Vortigern, only remaining member of the long-lived Pendragon bloodline, said to be descended from dragons. The first time king and soldier met was on the battlefield in only a brief skirmish. Their blades clashing despite Ambrosius' weakness in comparison to the near all-consuming might of the king before him. But the young soldier was spared , and slowly a fascination grew. Again and again they would meet in battle, only for both to walk away with little more than bruises. Until one battle where Ambrosius nearly met his end were it not for being found in the aftermath, barely alive, by Vortigern and his mage.

It was during the time he was healing in Vortigern's care as a "guest" that he learned why the man was so intrigued by Ambrosius, and to a lesser extent his younger brother who followed him: they were brothers. 

This knowledge would linger at the edge of the soldier's mind, flaming his disdain of the Empire and his desire to be free into an inferno. 

And so he followed his heart, and betrayed Rome. 

Following this decision, Ambrosius and Uther were fully taken in to Vortigern's court, where at Vortigern's gentle urging, Ambrosius took up a new name. "Ambrosius Aurelianus is dead, he burned with the rest of the fortress," the newly named Emrys Pendragon declared. And for the first time in his life, that star on the horizon felt closer. Like he had finally taken a proper step towards that small, bright light. 

A light that grew even closer when Merlin and Vortigern led him to a ruin where the sword Caledfwlch was sealed. 

None before had been able to fully approach the sword's resting place, even less touch it. But for Emrys it was as if nothing was in his way. And when he drew the sword from its scabbard for the first time since it was forged in the distant era when gods walked freely upon the earth, it was as if he was reunited with a piece of himself that he had lost.

Shortly following this Vortigern established the land of Camelot, and crowned Emrys as its first king. 

The years that followed were brief, in the long run, but those few years were a golden era for the newly formed kingdom. Alliances were forged, wars were quashed, and across Britain a peace began to form as High King of Britain Vortigern and his allies and brother helped to stabilize the land. But sadly such a golden era was doomed to end, and end it did when Uther's lust for the wife of Emrys' friend and allied king Gorlois, Igraine, led him to conspire with Merlin to make a decision that would rock the foundations of Britain. 

Discovering what Uther had done, Vortigern flew into a rage. Emrys, ever the peacekeeper between his brothers tried to intervene, and for his actions...

Was accidentally slain by Vortigern in what could have only been a chain of events choreographed by the hand of fate.

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As a Heroic Spirit, Emrys has been summoned as a Saber-class Servant. With Caledfwlch in hand, he practically dances across the battlefield, looking like a star streaking across the sky. While his Endurance is rather low due to poor health in his life, his other stats have suffered no such demerit.

"If you've summoned me, it was due to a need for an ally in your battle, correct? I swear upon my sword that I shall do all in my power to assist you, Master. Caledfwlch will carve a path to victory with its flames, and if need be I shall be the kindling for you. Ah, I just ask that you do not put me in any teams with Roman Servants. To put it simply... We don't get along."