Excerpted from The Young Watcher's Handbook:
In those times, the vampires and demons and Lamia and other creatures of the night were a plague upon Greece. Travelers on the roads at night were eaten as often as not, and even the people of the walled poleis began to fear. The queen of Athens at the time was an Amazon called Antiope, and was wife to famed Theseus. She appealed to Artemis, who holds the Amazons in special favor, to aid her people, and Artemis heard her plea. Apollo chose to assist his sister, for he is ever an enemy to the creatures of the night.
The brother and sister decided to send a hero to fight the night monsters. At Artemis' insistence, the hero was to be a girl, a daughter of the Athenians. Apollo thought this was just. Apollo and Artemis could not agree whether the Slayer was to be a creature of the sun or of the moon. Apollo won this first argument, pointing out to his sister that she could not both be a child of night and the enemy of the night. So the Slayer is a creature of sunlight. But she is fated to fight and die in darkness, and soon you will see why.
Together the brother and sister granted to the Slayer the strength to match the demons in single combat. Artemis gave her the skill to use weapons and the urge for the hunt. Apollo gave her the gift of healing, so that she might stay strong and beautiful despite many battles. When she turned eighteen, however, she would be sacrificed to the gods in return for the gifts given. Apollo argued against this, saying that only the barbarians sacrificed unwilling victims, but Artemis demanded her just payment. In this argument Apollo yielded, as he must, for he is a god of justice.
This is why we send the Slayer to her Cruciamentum, though it grieves us, to return her to the goddess in payment for her aid.
The first Slayer was to be the chosen from the best of the daughters of Athens. The girls competed for the honors, and the youngest daughter of the queen bested all her sisters in the games. She was chosen to be the first Slayer. When she reached the age of fourteen and her menarche, Apollo and Artemis visited her and blessed her with the gifts of the Slayers. They taught her well and sent her to fight.
Sineya was a magnificent Slayer, graceful and skilled in battle. She slew the children of Echidna in great number. Apollo watched her fight and was smitten with her, for she was beautiful as all Slayers are. But Apollo did not approach her, for he knew it would anger his sister.
One day Sineya came upon a satyr Amphelos. Amphelos found Sineya sleeping in the woods at dawn after a long hunt. She was beautiful, and Amphelos approached her to lie with her. Sineya awoke and in her surprise killed Amphelos. She left his body hung upon a tree as a warning to those would would trouble the Slayer. But Amphelos was beloved of Dionysus, who was angered and grieved by his death. He went to Artemis and demanded satisfaction for the death of his eromenos. Artemis persuaded him to wait until Sineya had completed her appointed time as the Slayer. On her eighteenth natal day she would be given to Dionysus to do as he will, for on that day her life was already forfeit.
This is how the children of Dionysus came to be our sworn enemies. Beware satyrs and succubi, creatures with horns, and creatures who play upon the flute, for they will harm us if they can.
Sineya earned much glory in her years as the Slayer, and the wilderness of Greece once again became safe. The people flourished. But the day of her eighteenth natal day approached. Apollo came to Sineya and offered her her life, for it grieved him to see such beauty slaughtered. In exchange for her life, she was to lie with him. She refused, being honorable and determined to fulfill her mother's agreement with Artemis. He came to her every night with the same offer, but she was steadfast, though she had fallen in love with the golden youth.
On the morning of her natal day, she was bound and carried to the temple of Artemis to be offered. Dionysus himself appeared to claim his due. At the sight of him, Sineya at last revolted. She was brave in battle, but feared to allow herself to be slaughtered like kine are slaughtered upon the altar. Dionysus approached her to claim her sacred virginity and then her life, but found she had broken her bonds. They fought, and destroyed the temple around them with the ferocity of their contest. But it would have gone badly for Sineya if Apollo had not intervened. He changed her into the form of a lioness, and she fled into the forest.
This is why we send the Slayer to her Cruciamentum unbound, with her weapons in her hand, that she may not fear as Sineya did but instead find her fate bravely in battle.
Apollo came to her in the forest in the form of a lion and wooed her. At last Sineya yielded to him. They lay together and found much joy with each other. But Artemis hunted the lioness in the forest, and found Sineya where she hid. Sineya had broken the terms of the bargain her mother had made. For this, Artemis cursed the line of Slayers to long for the sun but fight alone in darkness and die with the moon's face turned away from them. Apollo answered this curse with a blessing: the light of the sun would forever after strengthen the Slayers and harm their enemies.
Apollo comforted her, but Sineya soon tasted the bitter dregs of the cup he had given her, when all Athenians turned from her in fear of the goddess' anger. Sineya lived alone, fighting the vampires at night, and during the day hiding from her people. She soon discovered she had conceived by Apollo.
When Artemis heard of this, her anger was great, for Sineya had spurned the goddess when she spurned her virginity. Artemis cursed Sineya to die in childbirth. Her time came upon her early, and her pain was great. At the end of a week of suffering, she called upon Apollo, who came to her and cut the child from her. Despite all his arts of healing, Sineya died.
This is why the Slayer must never lie with men, but must prize her chastity. The anger of Artemis will be upon any Slayer who allows herself to be got with child. And it is why Artemis does not look upon you with favor.
When Sineya died, another girl was called. But this girl was alone, and fought without knowledge, and died quickly. And so did the Slayer who followed her.
Apollo took Sineya's son, named him Leandros, and gave him to Chiron to raise. Leandros was a true son of Apollo, and distinguished himself in the games and with the lyre. He studied diligently and learned all he must. Though he worked hard, fully twenty Slayers fought and died before he grew to young manhood. Then Apollo came to him and armed him with fine weapons. The god taught him all he need know to guide the Slayer, and commanded him to teach his sons and daughters as well. He and his line were to serve the Slayer, wherever she may be found.
Leandros was the first Watcher and the father of us all.