Artoria was born as the heir to the throne, the daughter of the King of Britain, Uther Pendragon, and the blue-eyed Igraine, the wife of Uther's servant, the Duke of Cornwall. The king, believing in Merlin's prophecy, yearned for the birth of his appointed successor, but the child born was a girl--not the one he desired.
Merlin placed Artoria in the care of Sir Ector, a simple and wise old knight. She was raised with Sir Kay as her brother, she acted as his squire and received training from him, while also doing other chores.
Once the day of prophecy arrived, knights and lords from around the country gathered to be selected as king. But, to deir surprise, the only thing prepared at the place of selection was a naked sword stuck in a stone with a golden inscription on the hilt reading "Whosoe'er pulleth out this sword of this stone is rightwise king born of England.", but none were able to pull it out. As they began jousting, Artoria neared the stone and reached out for the sword.
Merlin appeared before her to tell her her she would no longer be human upon taking hold of the sword, but she had been prepared for the fact that "becoming a king means no longer being human", that a king is someone who kills everyone to protect everyone. The sword was pulled out as if it were only natural to do so, and the area was filled with light.
She became something not human in that instant. The king's gender doesn't matter, and no one will care about the king's appearance or even notice it as long as the king acts like a king. Even if anybody did notice the king was female, it would not be an issue if she was a good king. This started the time of the king who would become a legend. She stopped aging at 15 years old, back when she pulled Caliburn from the stone.
Artoria led Britain from Camelot, and after becoming a feudal lord like her father, she became a king with many knights under her, including the esteemed Knights of the Round Table. Most knights praised their master's immortality as divine. Her battles were the acts of a god of war. She only knew victory for ten years and twelve battles while she ran through those days as the king.
Merlin used magecraft to turn her into a pseudo-male capable of producing sperm for an unknown duration of time. During this period, she was enchanted by her sister, Morgan le Fay, who took some of Artoria's sperm, developed it within her own ovaries, and gave birth to the homunculus and complete clone of Artoria, Mordred. She was born and raised without Artoria's knowledge, quickly growing up due to her accelerated aging, and she managed to join the Round Table through her own efforts and Morgan's recommendation.
Upon claiming her heritage and right to the throne, Artoria completely rejected Mordred, and refused to recognize her as an heir. Mordred believed the reason that she was not accepted was due to the hatred her father bore Morgan, and that no matter how much she worked to excel over others, she would forever be viewed as a tainted existence. The love she had for the king up until that point was so great that her rejection made her hatred burn.
Artoria married Guinevere, but it was only out of obligation to have a wife. The truth was that Lancelot, one of Artoria's closest friends, and Guinevere loved each other.
She strictly kept to the oath that a king is not human and that one cannot protect the people with human emotions. She punished people without a single mistake. Even after, or possibly because of, one of her knights murmured "King Arthur does not understand human feelings."
Lancelot's affair with Guinevere was eventually revealed through the plans of traitors who hated Camelot. Artoria did not see this unrighteous action as a betrayal, understanding Guinevere's sacrifice due to having concealed her gender. She wrote a letter of pardon to Lancelot and Guinevere, and gave them her blessings, saying that their actions "must have good cause". However, Lancelot was afraid and ran, bringing about a tragedy where no one was right and no one was wrong.
Resulting from that, the distrust of the Round Table towards the King spread, and the reigns of power in Camelot were seized once the King departed for the Rome expedition. Mordred became the leader of the rebellion. After the King returned, Mordred raged, claiming that only she was fit for the throne. The truth of the matter was that she only wanted to be accepted by Artoria. The conflict eventually led towards a final fight, the Battle of Camlann, where both armies were dying in heated battle.
The two faced each other on a hill of swords in the middle of a blazing battlefield. Blaming the situation on the King for not giving her the crown, she asked if the King hated the "son of Morgan" that much. Artoria replied, "Not once did I despise you. There was only one reason I would not give you the throne. You didn't have the capacity of a King." Mordred was eventually defeated in single combat, collapsing while pierced by the holy spear Rhongomyniad. Freed of the mask, with a face identical, she said "....Father" while reaching out with blood-soaked hands, but was not even granted that wish as she fell. Due to being bound by a strong curse, Mordred still swung her sword after her death, leaving a fatal wound on Artoria.
Her dying body was escorted to a holy isle by Sir Bedivere. Artoria ordered a grieving Bedivere to dispose of Excalibur by throwing it back to the Lady of the Lake; in her absence, she reflected on her personal failures, regretting her life as king. Before her last breath, she appealed to the world; in exchange for services as a Heroic Spirit, she asked to be given an opportunity seek the Holy Grail to save her country.
King Arthur will not die until she obtains the Holy Grail, also meaning that King Arthur cannot die. The result of obtaining the Holy Grail and fulfilling the contract are already determined. As she cannot be summoned unless something called the Holy Grail is present, her existence is proof of its existence.
She was then summoned by Kiritsugu and Shirou Emiya during the Fourth and Fifth Holy grail Wars, respectively.
A member of the infamous Black Spark mission, Support Sq. Saber tried to protect everyone during the mission. Most notably she bodyblocked an attack meant for Heavy Cpt. Zhang. After the signal to bomb the area was called, during their escape, Saber is pierced from behind and collapses, causing Zhang to take out the chryssalid in retribution to Saber having saved him before. What he didn't expect was for Saber to be corrupted and revive, killing Zhang with her own bare hands.